Allow to set SET/GET ratio

Change-Id: Ied37b6138c7a5430773b0ac467a5a22631e1e3b4
Reviewed-on: http://review.couchbase.org/26746
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>