MB-100: large value size mrw tests

Change-Id: I973811d8f46c2a9b734a46d60335e2f5a57e95da
Reviewed-on: http://review.couchbase.org/29133
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>