BUGFIX (I hope): immediate ack on unexpected ID

When the receiver receives a packet that does not have the
ID that is needed to progress, immediately send a
(probably duplicate) ACK to the sender

This behaviour is modified from before, when we would only
send an ACK if the ID of the received packet was different
than the current maximum ID held in the receive buffer + 1
(or equal to m_nextID if the receive buffer was emtpy)