Streamline the typing of the client side caching API. Some of the
methods are defining commands of type `str`, while in reality tuples
are being sent for those parameters.

Add client side cache tests for Sentinels. In order to make this work,
fix the sentinel configuration in the docker-compose stack.

Add a test for client side caching with a truly custom cache, not just
injecting our internal cache structure as custom.

Add a test for client side caching where two different types of commands
use the same key, to make sure they invalidate each others cached data.