NAKACK2: merge NakReceiverWindow and Retransmitter

Both NakReceiverWindow and Retransmitter use their own data structures to keep a list of messages received (NRW) and seqnos to be retransmitted (Retrasmitter). This is redundant and costly memory-wise.
I suggest let's merge the 2 classes, or at least let them share the data structure which keeps track of received messages.
Suggestion II: create a ring buffer with a (changeable) capacity that keeps track of received messages and messages to be retransmitted.