WIP: Refactor server database

[Krzmbrzl]

This PR completely rewrites the database handling on the server side. In so doing it cleans up and unifies the mess of the current implementation and adds proper unit testing support - in particular for data migration paths, which should provide an overall more reliable experience for users as the tests more or less guarantee that the data migration will work as intended. Additionally, since unit tests are run against all supported backends (SQLite, MySQL, PostgreSQL), all should work equally fine (at least from a functionality point of view).

As a bonus, the dependence on Qt for the database handling is completely removed. Instead, the new framework is built on SOCI.

This PR should address the following issues:

• DB transitions/migration system for server #4980 (data can now be exported to and imported from JSON)
• MySQL : SQL Error with emoji : how to log server id for this SQL Error or fix that ? #4717 (all database backends are not unit-tested for dealing with unicode characters)
• Murmur upgrade leaves database in a half-upgrade state while killing itself on other configuration issues #3795 (all operations on the database are now encapsulated in transactions)
• Auto increment on users.user_id #3666
• murmur database write performance #3588 (we are using a completely new backend and new code - hopefully this fixes the issue)
• Client does not detect corrupt mumble.sqlite db #3254 (again: transactions should prevent database corruption from happening - unless there is a bug in the code)
• backup/restore virtual servers #2960 (backups can now be done by explicitly exporting data to JSON - it's up to the user though. No automatic backup implemented)
• Channel Listener SQL Error: a foreign key constraint fails #6283 (we just have to assume that this is fixed - no way to check without a repro)
• Username left in qhUserIDCache after unregistration #6383
• Murmur 1.2.19 unable to recover from timeout on DB connection with DB on HA solution #3796 (we don't hardcode timeouts, so this was likely a Qt thing - new backend, new hope :D )

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