Redis: Decouple rspamd + netfilter from the rest

[rallisf1]

Summary

Currently you can sync fail2ban whitelists/blacklists between servers via the API (still needs a custom script and cron job), but it is almost impossible to sync spam/ham.

This is due to a design flaw in mailcow that doesn't allow for multi-server redis replication, as all services use the same redis database. In such a scenario, as opposed to a hot standby setup, things like mailcow settings and domain data should not be replicated.

My proposal is to use a second Redis container just for netfilter and rspamd, which can then be replicated across multiple instances.

Motivation

This will enable having the same level of security (at least regarding firewall and spam protection) across someone's fleet of mailcow servers. Moreover; it will make management easier as admins will be able to ban/unban/whitelist an IP through any of the replicated mailcow instances.

Additional context

Would be happy to contribute code or sponsor.