uma: Add UMA_ZONE_UNMANAGED

Allow a zone to opt out of cache size management. In particular,
uma_reclaim() and uma_reclaim_domain() will not reclaim any memory from
the zone, nor will uma_timeout() purge cached items if the zone is idle.
This effectively means that the zone consumer has control over when
items are reclaimed from the cache. In particular, uma_zone_reclaim()
will still reclaim cached items from an unmanaged zone.

Reviewed by: hselasky, kib
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34142