Use per-domain locks for the bucket cache.

This gives much better concurrency when there are a large number of
cores per-domain and multiple domains. Avoid taking the lock entirely
if it will not be productive. ROUNDROBIN domains will have mixed
memory in each domain and will load balance to all domains.

While here refactor the zone/domain separation and bucket limits to
simplify callers.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D23673