MB-11731 remove shard level locking from ExecutorPool

we no longer need to block threads at shard level since
locks are acquired at the vbucket level

Change-Id: Iebc368d243499b76b5cf1f4f0d50c487866960dc
Reviewed-on: http://review.couchbase.org/39502
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>