Remember currently joined channels upon core exit
Especially in a multi-user environment it is important that the core can be restarted without requiring every user to rejoin the channels he was in. This means that the core needs to remember the networks everybody is connected to and the channels he joined when it exits, and it needs to reconnect/rejoin when it is started again. That way, the admin can easily restart the core for maintenance and whatever, with minimal disruption for users.
The Quassel Core now remembers on exit which networks where connected and which channels
everybody has joined, and restores this information upon restart (so all networks will
automagically connected and the appropriate channels rejoined).
Has not been tested with more than one active CoreSessions.
You can start the core without session restore by using --norestore as parameter.
Closes BR #63.