This acts as an async wrapper around `LruCache` making it possible to
build new cache classes that perform asynchronous IO with external systems.

This doesn't change function of the cache at all but prepares for the
possibility of external backed, or combination external/local backed,
event cache.

This does separate the cache invalidation into two options: the local
in memory cache and any asynchronous cache (which is currently the
same thing). This means that the worker invalidating objects on changes
must call the async method and the local method is reserved for
invalidations received over replication.

Only used in tests but now is consistent with the rest of the API.