Use GetMulti to reduce the number of requests to memcached

use getMulti to reduce the number of requests to the cache backend. This works automatically for tags and tagged caches; additionally, keys staged with stage() will be gotten on the next get, and subsequently those keys can be gotten with get() without triggering an additional request to the back end. This is an experimental feature.
Full Changelog: v2.0.3...v3.0.0