tcp: SACK Lost Retransmission Detection (LRD)

Recover from excessive losses without reverting to a
retransmission timeout (RTO). Disabled by default, enable
with sysctl net.inet.tcp.do_lrd=1

Reviewed By: transport, rrs, tuexen, manpages
Sponsored by: Netapp, Inc.
Differential Revision: https://reviews.freebsd.org/D28931