tcp: do not purge SACK scoreboard on first RTO

Keeping the SACK scoreboard intact after the first RTO
and retransmitting all data anew only on subsequent RTOs
allows a more timely and efficient loss recovery under
many adverse cirumstances.

Reviewed By: tuexen, transport
MFC after: 10 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42906