Instead of always performing the periodic integrity check, make it possible
to disable it or run it at different intervals.
Currently for the Blender Studio it's crunch time, so the check should
really only run when there is someone looking at the system (i.e. at
restarts for upgrade purposes).
Perform these two SQL calls & check their result:
- `PRAGMA integrity_check`
- `PRAGMA foreign_key_check`:
See https: //www.sqlite.org/pragma.html for more info on these.
This also removes the unused `PeriodicMaintenanceLoop()` function.
Periodic checking while Flamenco Manager is running might be introduced
in a future commit, after the startup-time checks have been shown to not
get in the way.
