firewall: use incremental updates of udb/mdb

[Rémi Audebert]

Currently firewall.py does a query to udb and mdb at each presenced update. It would be a lot better to also poll udb and mdb in order to maintain a local database of users and machines.

[Antoine Pietri]

This cannot be done easily because it requires to call poll_updates() multiple times which runs forever as the main loop of the program. Maybe we should use either run_in_executor (easy) or rewrite all the code in an async fashion (hard)?