tcp: include RFC6675 IsLost() in pipe calculation

Add more accounting while processing SACK data, to
keep track of when a packet is deemed lost using
the RFC6675 guidance.

Together with PRR (RFC6972) this allows a sender to
retransmit presumed lost packets faster, and loss
recovery to complete earlier.

Reviewed By: cc, rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D39299