MB-100: new mrw confs for large key sizes

Change-Id: Ie329a87389d91e424ab9753c3d39382cc6dbae72
Reviewed-on: http://review.couchbase.org/29118
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>