Clear MUC data on disconnection

Avoid "phantom" participants when we reconnect and rejoin an MUC room.
Prepare to observe more than one status code in MUC presence
Note if newly created MUC room is locked to others
Be more clever about identifying self-presence for MUC

Presence stanzas that concern us have status code 110.  This is
particularly important to notice when the server changes our nick for
us (status code 210).