upgrades: upgrade makes Quassel-Core keeps popping in and out in connected IRC channels during the upgrade period

This issue was raised at #freedombox IRC channel and answered.

• jvalleroy: I think this is an issue for FreedomBox, and we should try to solve it.
• jvalleroy: It can happen during any package upgrade, but it will be much more noticeable during a dist-upgrade.
• jvalleroy: I think it's due to interaction between unattended-upgrades and needrestart.