MB-12067 [jemalloc]: use one arena

Use just one arena, instead of the default based on number of
CPUs. Helps to minimize heap fragmentation.

Change-Id: I5b55192d41c4ce5d8bb59c9063f3698a12909248
Reviewed-on: http://review.couchbase.org/42889
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>