[tcp] add PRR 6937bis heuristic and retire prr_conservative sysctl

Improve Proportional Rate Reduction (RFC6937) by using a
heuristic, which automatically chooses between
conservative CRB and more aggressive SSRB modes.
Only when snd_una advances (a partial ACK), SSRB may be
used. Also, that ACK must not have any indication of
ongoing loss - using the addition of new holes into the
scoreboard as proxy for such an event.

MFC after: 4 weeks
Reviewed By: transport, kbowling, rrs
Sponsored By: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28822