Without the ability to retrieve underlying items by key (instead of
the object passed to KeyFunc) it is impossible to build wrappers
around cache.Store that want to make decisions about keys, because
they would need to reconstruct the object passed to Get.
This allows retrieval by key, and makes sure Get(obj) uses it.
Support namespacing in cache.Store by framing the interface functions
around interface{} and providing a key function to each Store implementation.
Implementation of a fix for #2294.
