MB-100: mixed workload with 2 buckets

Change-Id: I82c5f481dabaefa63eeb21f1729cdb06e5b31259
Reviewed-on: http://review.couchbase.org/24508
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>