In order to migrate a production Redis store (which holds user sessions) to another server without downtime, we should allow configuring DATASTORE_REDIS_ARGS to return an array of connection details which musicbrainz-server will distribute writes across.  (Reads will just return a value from the first server which has the requested key.)

This feature is only useful to avoid downtime in production where a single standalone Redis instance is used.  It's not intended as an alternative to a Redis cluster.