public synchronized List<QueuedStanza> purgeQueue() {
            Log.trace( "Purging queue (size: {}) of stanzas to be sent to peer as soon as federation has been established.", queue.size() );
            final List<QueuedStanza> result = new ArrayList<>( queue );
            queue.clear();
            return result;
        }
Retrieve and clean the list of stanzas that have been queued after federation was initiated, but before it
was finished.

@return A list of queued stanzas (possibly empty).
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/FMUCHandler.java