MB-10027 - remove task mutex from GlobalTask

Replace with AtomicValue
This is meant to be the first of multiple changes to reduce
the locking overhead of the Global Thread Pool modules

Change-Id: I4f8cd42a786a389b36580d05adfbf327537ff9a5
Reviewed-on: http://review.couchbase.org/32976
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>