instead of rely on ListenerWrapper checking the PacketFilter *and*
invoking the PacketListener we now use two for-each loops, where the
first filters the PacketListeners that should get invoked and use the
second for-each loop to actually invoke the PacketListener.
Before, the code was not thread safe if a PacketListener would remove
itself from the (send|recv)Listeners.
Also make packet(Listener|Filter) in ListenerWrapper final.