many jabber queue management changes

many jabber queue management changes

Added a method to QueueManager to let subclasses do stuff when idle.
Needed so that XMPP queue manager can service its message queue.

Cleaned up jabber_broadcast_message quite a bit. Use custom joins
instead of loop-and-query, should fix some problems with users who are
getting messages even after turning off notification. Only build $msg
and $entry once, and use the XMPPHP function for messages with a
payload, rather than rolling our own.

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