More XMPP fixes. The most important change is that it now stores important
messages, like push requests, and (re)sends them when a buddy's client
sends XMPP presence. This makes XMPP syncing much more robust, all the
clients do not need to already be connected when messages are initially
sent, but can come and go. Also fixed a bug preventing syncing from working
immediately after XMPP pairing. XMPP seems to be working well now; I only
know of one minor bug.
