Add congestion control framework to link.
Change queue to deque.
Add retransmit timeout handler.

It now causes simulation to run endlessly.
Change to debug loglevel.
Lazily ack the received ack packet.
This section needs testing.
Add some congestion control initialization.
Add channel::stop().
Add channel::may_transmit().

With some rudimentary congestion control code.
Textual cleanup.