Remove register from queue only when new value is seen
ack_last_sdp() can be used to send settings queue

TODO: split ACK and queue preparation and datagram sending
Split _update_state_from_sdp() from _process_incoming_sdp()
Add some SDP datagramm timestamp cheks
Change log level
FIX: do not iterate members keys if there is no members
Add __getstate__() for pickling
Value is required
Remove of missing member does not raise an exception any more
SDP.decode() may fail, catch exception