multiparty: implement rejoin after connection re-establishment
This completes the rejoin implementation. Yeah!
It triggered quite an amount of rework:
* the ID for multiparty chat sessions is now the roster manager, not the
call ID. That made the roster_manager field in sip_session unnecessary.
* code cleanup in process_incoming_invite(). At least the parts relevant
to multiparty should be much clearer now.
As I don't have access to a 2005 server I can't test the multiparty changes.
I can only hope that I didn't break it...