fix: re-implement lock for DBMixin

[ClemDoum]

Bug description

On some OS, there seem to be race conditions in the JSON DB serialized on the filesystem leading to failures such as this one.

To avoid read/writing to the DB at the same time, the DBMixin should read/write with lock.
This behavior was implemented earlier with a thread.Lock but has been remove has the lock is not serializable accross processes and hence don't worker properly with multiprocessing workers.

Leveraing filelock could be a potential fix for this issue.