Re-add support for memory_store

PR #154 removed support for `memory_store`, due to the requirement for `cas`/`cas_multi`.

This adds a `MemoryFetcher` ands uses it instead of `CacheFetcher` if the `cache_backend` doesn't respond to `cas` and `cas_multi`.

Fixes #160 

@arthurnn @dylanahsmith for review /cc @camilo