tcp: fix duplicate retransmissions when RTO happens during SACK loss recovery

When snd_nxt doesn't track snd_max, partial SACK ACKs may elicit
unexpected duplicate retransmissions. This is usually masked by
LRO not necessarily ACKing every individual segment, and prior
to RFC6675 SACK loss recovery, harder to trigger even when an
RTO happens while SACK loss recovery is ongoing.

Address this by improving the logic when to start a SACK loss recovery
and how to deal with a RTO, as well as improvements to the adjusted
congestion window during transmission selection.

Reviewed By: tuexen, cc, transport
Sponsored by: NetApp, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43355