Reset the retransmission timer when receiving duplicate ACKs.

While we are getting duplicate ACKs, and are likely in fast recovery,
keep resetting the retransmission timer, so that if fast recovery takes
longer than the RTO, we don't trigger a slow start.

When we do get a real timeout during fast recovery, reset the duplicate
ACK counter to signal we are no longer in fast recovery.

We also should not do anything when receiving duplicate ACKs when all
data has already been ACKed.