MB-7965: Unit test to check behavior with multiple flush requests

Supporting synchronous bucket flush operation

Change-Id: I5d895b2b57bcec01ffb556ed635771445ef0eead
Reviewed-on: http://review.couchbase.org/43976
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>