MB-10539-debug: avoid ExecutorPool mutex to snooze in bgfetcher

Use of ExecutorPool->snooze() unnecessarily increases lock
contention within the shared thread pool in bgfetcher

Change-Id: Ia0a0fed93f170d6e504a24f54c2907b32d7851a2
Reviewed-on: http://review.couchbase.org/35577
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>