Reduce "thread.arena" mallctl contention.
Reduce "thread.arena" mallctl contention.

This resolves #460.

wqfish/jemalloc,8l/jemalloc,8l/jemalloc,wqfish/jemalloc,8l/jemalloc,wqfish/jemalloc,8l/jemalloc,wqfish/jemalloc,wqfish/jemalloc