enh(metadata): store current file etag

[ArtificialOwl]

This PR adds a way to verify if a metadata is up-to-date
This is done by storing the current file etag when setting a new value for a metadata:

{
   "mymeta": {
     "value": "this is a test",
     "type": "string",
     "etag": "abcd1234",
     "indexed": false,
     "editPermission": 1
   },
   "myapp-anothermeta": {
     "value": 42,
     "type": "int",
     "etag": "0987zyxw",
     "indexed": true,
     "editPermission": 0
   }

When called, the method isUpToDate($metadataKey) will compare current file etag with the one stored aside the metadata

(while not mandatory, can be backported to 28)