Add a Redis persistor to allow in-memory caching in a cluster or serverless environment

• add redis type and --cache-url/--inputcache-url to bin/caravaggio script
• add optional init() and shutdown() methods for persistors to allow to eg. connect/disconnect to a server
• do not implement flush() - a Redis cache likely is shared across services/applications.
• tests don't verify expiration/purging, as this is Redis' duty