NCBC-190: Support bucket flushing

Change-Id: Ie945e6c36ad9936684d78b67e0eb260997f33014
Reviewed-on: http://review.couchbase.org/23660
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>