MFC r349893:

This commit updates rack to what is basically being used at NF as
well as sets in some of the groundwork for committing BBR. The
hpts system is updated as well as some other needed utilities
for the entrance of BBR. This is actually part 1 of 3 more
needed commits which will finally complete with BBRv1 being
added as a new tcp stack.

Merge conflics were manually resolved.

Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D20834