flush multi vbucket unit test fix - acquire vbucket lock for flushall

failure to acquire this lock can cause flusher and
flushall tasks to race and hit the assert highSeqno <= vb->getHighSeqno()

Change-Id: Ifd78642a27c76f07a1c94aabb2b7963d84efc053
Reviewed-on: http://review.couchbase.org/40029
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>