Move image caching into the store rather than a middleware

Currently glance image caching lives in a wsgi middleware. This design requires us to duplicate important business logic like notifications and serialization all for the sake of caching image data. A more natural place to accomplish this caching would be to put a cache on top of the store layer, so that a call to store.get_from_backend(location) would check the local cache first before actually calling out to a remote store.