A pair of performance improvements.

Swap buckets on free as well as alloc so that alloc is always the most
cache-hot data.

When selecting a zone domain for the round-robin bucket cache use the
local domain unless there is a severe imbalance. This does not affinitize
memory, only locks and queues.

Reviewed by: markj, rlibby
Differential Revision: https://reviews.freebsd.org/D23824