Mint: allow caching with external Redis service

[callebtc]

Currently, cached request/response pairs are written to memory (#361). It would be great if the mint could use a dedicated Redis caching service for this (as well or instead of RAM caching).

[mubarak23]

A setup that set memory as default for cashing requests/ response,
But allow mint to setup their own external
Redis caching service

[thesimplekid]

I think this is a good idea and something that can be supported fairly easier by changing the cache to be a Cache trait instead of a specific struct like we do for the database within the mint.

[crodas]

@thesimplekid I'm working on this, upcoming PR. My approach is to always have an in memory cache and use redis to synchronize cache between nodes /cc @callebtc