UDP improve accept behaviour of incoming packets

[hannesm]

each packet carries a packet_id, and the current implementation requires it to be strictly the next one of the last received. now, in UDP packets may arrive out of order, and if this happens, we discarded all future packets (i.e. not sending back an ack, not storing them anywhere); and wait for the exact next packet (with packet_id being last_received_packet_id + 1). A more optimal strategy could be to accept future packets in a window, ack them, and carry some data structure (list?) with the holes of the packet_id space.

related is the handling of old packet_id: assume the peer send us 5, we acked, they send us 6, and 5 again (since the ACK was lost on the way). currently, we ignore/drop the re-sent 5; whereas a well-behaving implementation should likely re-send the ACK for 5.

[hannesm]

#148 is more accurate, so I'll close this issue.