Store the channel keys in CoreNetwork again

Store the channel keys in CoreNetwork again

The refactoring from 3146ad0 (and e0781ee) was not a true refactoring as
it changed behaviour: CoreIrcChannels (as opposed to CoreNetworks) are
ephemeral, which meant disconnecting or parting lost the channel key.
Keys are still kept by the channel's cipher, but synced from/to its network when
the constructor/destructor is called.

Fixes #1248