MB-100: enforce using READ ops for cache warmup

Change-Id: I27893bdf0416e41a4fe99b2ff4a44f4cff0337b5
Reviewed-on: http://review.couchbase.org/28361
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>