update the SACK loss recovery to RFC6675, with the following new features:
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Description

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
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D18985