Bind TCP HPTS (pacer) threads to NUMA domains

Bind the TCP pacer threads to NUMA domains and build per-domain
pacer-thread lookup tables. These tables allow us to use the
inpcb's NUMA domain information to match an inpcb with a pacer
thread on the same domain.

The motivation for this is to keep the TCP connection local to a
NUMA domain as much as possible.

Thanks to jhb for pre-reviewing an earlier version of the patch.

Reviewed by: rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20134