mod_smacks: Limit queue memory consumption using new util
This brings back the queue size limit that was once added, then removed
because destroying the session when reaching the limit was not great.
Instead, the queue wraps and overwrites the oldest unacked stanza on the
assumption that it will probably be acked anyway and thus does not need
to be delivered. If those discarded stanzas turn out to be needed on
resumption then the resumption fails.
Copyright (c) 2008-2011 Matthew Wild
Copyright (c) 2008-2011 Waqas Hussain
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