ref(redis): Use a readonly client for project configs [INGEST-1557]

[jan-auer]

We use a single Redis client for both rate limiting and fetching project configs. Since the former needs to modify keys, the client cannot be set to readonly mode. Project config fetching only reads keys from redis, so setting the cluster client to readonly mode allows it to read from replicas, too.

Note - This PR is not yet merge ready:

• There should be a single Redis client in non-cluster mode
• Redis pool initialization should be moved and deduplicated
• Changes from master need to be merged in
• Cluster connections aren't configurable for single mode.

Based off of 1d91c35, which is currently deployed.

See INC-192

[jan-auer]

This is much less urgent after #1394. Both of the changes in this PR may still be useful, although there's open questions on how to expose the config and whether it's desirable to have separate connection pools.