XEP-0198: Terminate session if stanza queue becomes too large

The XEP-0198 code saves a copy of each incoming stanza until the stanza is acknowledged by the client.  However, the size of the stanza queue must be limited to make sure the server won't be liable to DoS.  Currently, the oldest item is dropped from the queue if the queue length reaches some threshold (configurable via `max_ack_queue`).  This behavior renders stream management unreliable: Stanzas are dropped without a bounce, and if the session is later [resumed][1], the client might miss (some of) the dropped stanzas.  Therefore, it's [probably better][2] to terminate the session on queue overflow.

This patch implements that by sending a `mgmt_queue_full` message to `self()`, because propagating the termination up the call stack seems a bit cumbersome.  If you don't like that, please let me know.

[1]: http://xmpp.org/extensions/xep-0198.html#resumption
[2]: http://mail.jabber.org/pipermail/standards/2014-May/thread.html#28842