One on one chat fixes

This causes messages from individual users (outside of MUCs) to be displayed in a single tab per bare JID, while messages we send are directed appropriately as per http://xmpp.org/rfcs/rfc6121.html#message-chat.

Todo:

* [x] Expire resource lock when we receive presence from the user at any resource
* [ ] Expire resource lock after some timeout ?

A follow-up PR will permit linking a MUC roster entry to these standard 1-on-1 chats when we know their real JID.