Support TransactionMessageAttempts and -RetryInterval

[matth-x]

This PR adds the implementation of the configurations TransactionMessageAttempts and -RetryInterval.

For these configs, the RequestQueue implementation needed to be refactored. This changes the following:

• the Request store (including the files opstore.jsn and op-X.jsn on flash) is no longer used. Furthermore, the auxiliary file txstore.jsn of the transaction store was removed
• the temporal order of outgoing OCPP operations is not enforced anymore, except for tx-related messages. This means that the StatusNotification "Finishing" could arrive before the StopTransaction for example
• the compact messageIds (starting with "1000000") are not longer beneficial storage-wise. MicroOCPP uses pseudo-GUIDs from now on as it is the market standard

[luca-eidos]

Hi @matth-x, first of all, thank you so much for your effort.
I've just updated MicroOcpp in my project to the latest version. With this edit, if a reset occurs mid-transaction for whatever reason, I find it no longer possible to recover, while in the past it was. Am I missing something? Thank you!

[matth-x]

Hi @luca-eidos, thanks for reporting this. The general scenario of resetting the charger during the transaction should be covered by the unit tests, so I assume that this is a more special case which is not yet considered. Can you give more details about the error scenario, e.g. if the charger is offline, you see a boot loop, messages are not going through at all, etc.?

[luca-eidos]

Hi @matth-x, thank you for your response. Actually, I was wrong about this issue, it works perfectly fine. As tx-store.json was used in the previous version, I thought this feature was no longer available.
I've just found it was an issue with my custom implementation of ftw_root, passing the absolute path instead of the relative to fn(). Thank you again for your kind reply and your precious work, wish you a wonderful day!