Standardize on MomentoResult<Success, Error> and MomentoResult <Hit, Miss, Error> for responses

[nand4011]

Cache client responses should all be a MomentoResult that returns a MomentoError or a response object. The response should be a Hit that contains the response value or Miss containing nothing, or a plain response if a miss cannot occur.

cache_set_fetch_response and cache_sorted_set_fetch_response currently return Optional values instead of being a hit or miss.