Add XMPPConnection.createPacketcollectorAndSend(PacketFilter, Packet)

Using

createPacketCollector(filter);
sendPacket(packet);

was error prone, i.e. the PacketCollector could leak if sendPacket()
would throw an exception and the user forgot to call
PacketCollector.cancel(). For cases where
createPacketCollectorAndSend(IQ) is not sufficient (because we don't
send IQs), createPacketCollectorAndSend(PacketFilter, Packet) is now
used, which does take care that the PacketCollector does not leak if
sendPacket() throws an Exception.
Rework MultiUserChat

remove PacketMultiplexListener, RoomListenerMultiplexor and
ConnectionDetachedPacketCollector(Test), which was a bunch of
(in same cases redundant) code that formed a complex construct that
presumably tried to make MultiUserChat instances easily garbage
collect-able.

Now, MultiUserChat should be eligible for gc if the userHashLeft() is
invoked before the reference to the instance is dropped, which should be
the case in the most scenarios. Otherwise the connection may references
the MultiUserChat instance over Packet(Listener|Interceptor)s preventing
the gc.