Add concept of connectors to cacheables

[bwees]

This PR allows a custom cache connector to replace the in-memory cache. This PR retains the default behavior of the library (in-memory cache) but allows you to specify a connector that implements CacheConnector. This will allow users to implement databases such as Redis or other KV stores as cache storage.

The in-memory cache has been broken into its own class that implements CacheConnector to simplify the code. This class is used by default if a connector is not specified.