update the SACK loss recovery to RFC6675, with the following new features:
- improved pipe calculation which does not degrade under heavy loss
- engaging in Loss Recovery earlier under adverse conditions
- Rescue Retransmission in case some of the trailing packets of a request got lost
All above changes are toggled with the sysctl "rfc6675_pipe" (disabled by default).
Reviewers: transport, tuexen, lstewart, slavash, jtl, hselasky, kib, rgrimes, chengc_netapp.com, thj, manpages, kbowling, NetApp, rscheff
Reviewed By: transport
Subscribers: imp, melifaro
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D18985