Reduce Redis traffic between workers.

[erikjohnston]

On matrix.org or we're seeing a lot of traffic going over our VPNs, most of which is Redis related. We should try and reduce this a bit.

Some things that would help, in order of effort:

• Add support for TLS when connecting to Redis #12462 ─ This would allow connecting directly rather than over the VPN.
• Move USER_IP replication command to a separate redis pub/sub channel. #12460
• Split different RDATA streams into different redis channels #12461