thread: numa-aware zombie reaping

The current global list is a significant problem, in particular induces a lot
of cross-domain thread frees. When running poudriere on a 2 domain box about
half of all frees were of that nature.

Patch below introduces per-domain thread data containing zombie lists and
domain-aware reaping. By default it only reaps from the current domain, only
reaping from others if there is free TID shortage.

A dedicated callout is introduced to reap lingering threads if there happens
to be no activity.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D27185