Reduce client connection pool size for Redis Ratelimiting

We might be opening up too much connections from the client's connection pool to the Redis Ratelimiting cluster.

The current pool size is 14 on web and concurrency + 5 on sidekiq (depending on the shard).

This means we want to support configuring the pool size per Redis instance basis and reduce the size for Redis Ratelimiting.

This ticket was created from INC-2673 and was automatically exported by incident.io 🔥