MB-9015 Return the items CAS value from touch

Change-Id: Ie9a5d3f8a520c7f7553cfa990270b1464534ce78
Reviewed-on: http://review.couchbase.org/28569
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>