MB-11287 Lower default atomic operation ordering

Lowering global default memory ordering stringency to
memory_order_acq_rel to avoid costly CPU fence operations

Change-Id: Ic8f14abc7843f931242c61ba98c5cc667f52023d
Reviewed-on: http://review.couchbase.org/37981
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>