MB-11287 use less stringent memory_order for TaskQueue lock

The default memory_order for the atomic variable isLock in
TaskQueue is sequential ordering which imposes severe CPU
fence and results in high CPU usage on machines with many cores.

The acquire-release ordering should offer a balanced mix of
ordering and performance for this use-case.
http://en.cppreference.com/w/cpp/atomic/memory_order

Change-Id: Ida12cfaf428b725f5a960f4f34e527f548e01c91
Reviewed-on: http://review.couchbase.org/37976
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>