set up 2 instances of redis: one for caching frequently requested data, one for persistent storage

[danielblagy]

Why:

• we should cache frequently requested data (queried from postgres db)

• we need a persistent storage for blacklisting jwt tokens for security

• set up 2 redis instances in docker-compose, configure one instance to be an LRU cache, another for persistent storage with saving to disk

• refactor internal/service/cache

• create a persistent store service

[danielblagy]

FOR QA:

1. Set up 2 redis instances, one for caching, one for persistent storage.
2. If user is not logged in, 401 Unauthorized is returned instead of 400 BadRequest.