Frequent error 502 and high server load since update to 16.0.1

### Steps to reproduce
1. Upgrade to 16.0.1

### Expected behaviour
Load should be normal and no 502 error

### Actual behaviour
From time to time Nextcloud becomes unavailable with a 502 - Bad Gateway error and the load is between 10 - 15. 
I checked the processlist of MariaDB/MySQL and there seem to be a lot of queries going on for many hours:
<details>
<summary>MySQL processlist</summary>

```
+--------+--------------+-----------+--------------+---------+------+----------------+------------------------------------------------------------------------------------------------------+----------+
| Id     | User         | Host      | db           | Command | Time | State          | Info                                                                                                 | Progress |
+--------+--------------+-----------+--------------+---------+------+----------------+------------------------------------------------------------------------------------------------------+----------+
| 324404 | <REMOVED> | localhost | <REMOVED> | Query   |  187 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 23, 1, 8, 1, 1, 30)) AND ((`f`.`mim |    0.000 |
| 326496 | <REMOVED> | localhost | <REMOVED> | Query   |  200 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 23, 1, 8, 1, 1, 30)) AND ((`f`.`mim |    0.000 |
| 334566 | <REMOVED> | localhost | <REMOVED> | Query   |   99 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 165)) AND ((`f`.`mimetype` <> 2) OR |    0.000 |
| 334646 | <REMOVED> | localhost | <REMOVED> | Query   |   99 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 165)) AND ((`f`.`mimetype` <> 2) OR |    0.000 |
| 334847 | <REMOVED> | localhost | <REMOVED> | Query   |   67 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 20, 1, 1, 55)) AND ((`f`.`mimetype` |    0.000 |
| 339223 | <REMOVED> | localhost | <REMOVED> | Query   |   53 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 88)) AND ((`f`.`mimetype` <> 2) OR  |    0.000 |
| 341354 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341418 | <REMOVED> | localhost | <REMOVED> | Query   |   51 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341527 | <REMOVED> | localhost | <REMOVED> | Query   |   51 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341617 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341643 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341685 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341714 | <REMOVED> | localhost | <REMOVED> | Query   |   54 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341730 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341835 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341875 | <REMOVED> | localhost | <REMOVED> | Query   |   51 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 341938 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 342183 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
| 343104 | <REMOVED> | localhost | <REMOVED> | Query   |  204 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (1, 8, 1, 1, 224)) AND ((`f`.`mimetype` |    0.000 |
| 343386 | <REMOVED> | localhost | <REMOVED> | Query   |   20 | Sorting result | SELECT `f`.* FROM `oc_filecache` `f` WHERE (`f`.`storage` IN (20, 1, 8, 1, 1, 61)) AND ((`f`.`mimety |    0.000 |
```
</details
The installation has around 800'000 files, which may have an influence on the speed of these queries, but it has worked in the past.

### Server configuration

**Operating system**:
Debian 9.9
**Web server:**
Nginx
**Database:**
MariaDB 10.1
**PHP version:**
7.2
**Nextcloud version:** (see Nextcloud admin page)
16.0.1
**Updated from an older Nextcloud/ownCloud or fresh install:**
Updated from 13.0.4
**Where did you install Nextcloud from:**
Website

**List of activated apps:**
<details>
<summary>App list</summary>

```
Enabled:
  - accessibility: 1.2.0
  - activity: 2.9.1
  - announcementcenter: 3.5.1
  - apporder: 0.7.1
  - bookmarks: 1.0.6
  - bruteforcesettings: 1.3.0
  - calendar: 1.7.0
  - circles: 0.17.3
  - cloud_federation_api: 0.2.0
  - comments: 1.6.0
  - contacts: 3.1.3
  - dav: 1.9.2
  - encryption: 2.4.0
  - external: 3.3.0
  - federatedfilesharing: 1.6.0
  - federation: 1.6.0
  - files: 1.11.0
  - files_pdfviewer: 1.5.0
  - files_rightclick: 0.13.0
  - files_sharing: 1.8.0
  - files_texteditor: 2.8.0
  - files_trashbin: 1.6.0
  - files_versions: 1.9.0
  - files_videoplayer: 1.5.0
  - firstrunwizard: 2.5.0
  - gallery: 18.3.0
  - logreader: 2.1.0
  - lookup_server_connector: 1.4.0
  - music: 0.9.5
  - nextcloud_announcements: 1.5.0
  - notifications: 2.4.1
  - oauth2: 1.4.2
  - password_policy: 1.6.0
  - privacy: 1.0.0
  - provisioning_api: 1.6.0
  - quota_warning: 1.5.0
  - radio: 0.6.5
  - recommendations: 0.4.0
  - serverinfo: 1.6.0
  - sharebymail: 1.6.0
  - support: 1.0.0
  - survey_client: 1.4.0
  - systemtags: 1.6.0
  - theming: 1.7.0
  - twofactor_backupcodes: 1.5.0
  - updatenotification: 1.6.0
  - viewer: 1.0.0
  - workflowengine: 1.6.0
Disabled:
  - admin_audit
  - files_external
  - user_ldapr
```
</details>

**Nextcloud configuration:**
<details>
<summary>Config report</summary>

```
{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "16.0.1.1",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "logtimezone": "Europe\/Zurich",
        "installed": true,
        "activity_expire_days": 180,
        "trashbin_retention_obligation": "auto,90",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "loglevel": 2,
        "log_type": "errorlog",
        "mail_smtpmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpauth": 1,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "25",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "updater.release.channel": "stable",
        "maintenance": false,
        "theme": ""
    }
}
```
</details>

**Are you using external storage, if yes which one:** local/smb/sftp/...
no
**Are you using encryption:** yes/no
yes
**Are you using an external user-backend, if yes which one:** LDAP/ActiveDirectory/Webdav/...
no

### Logs

Nothing interesting in the logs.


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Frequent error 502 and high server load since update to 16.0.1 #16020

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Uh oh!

Frequent error 502 and high server load since update to 16.0.1 #16020

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