tcp: refactor cwnd during SACK transmissions to allow TSO

Refactoring of cwnd and moving the adjustment for SACKed data into
tcp_output() - cwnd tracking the maximum extent starting at snd_una -
allows both SACK loss recovery as well as SACK transmissions after
RTO during slow start and if allowed, the use of TSO while in loss
recovery.

Reviewed By: tuexen, cc, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43470