Memcache mergeable - use semigroup

Addresses https://github.com/twitter/storehaus/issues/250

There is a cas-based mergeable memcache store. Adding a hook for that as well might help folks try it out. Will send that in a separate change.
https://github.com/twitter/storehaus/blob/develop/storehaus-memcache/src/main/scala/com/twitter/storehaus/memcache/MergeableMemcacheStore.scala