The previous model was flawed. Consider the following case:
* The main loop looks as follows (pseudocode):
loop {
let _ = (tcp:1234).read_all()
wait(iface.poll())
}
* The remote end is continuously transmitting data and at some
point fills the window of (tcp:1234), stopping the transmission
afterwards.
* The local end processes the packets and, as a part of egress
routine, emits an ACK. That also updates the window, and
the socket's poll_at() routine returns None, since there is
nothing to transmit or retransmit.
* The local end now waits indefinitely even though it can start
processing the data in the socket buffers right now.
