Retransmitter task which periodically (every xmit_interval ms):
 
 - If any of the receiver windows have the ack flag set, clears the flag and sends an ack for the
 highest delivered seqno to the sender
- Checks all receiver windows for missing messages and asks senders for retransmission
- For all sender windows, checks if highest acked (HA) < highest sent (HS). If not, and HA/HS is the same
 as on the last retransmission run, send the highest sent message again