OpenDKIM has not been updated in the last 7 years and failed to adopt
RFC8463, which introduces Ed25519-SHA256 signatures.
It has thereby held back the DKIM ecosystem, which relies on the DNS
system to publish its public keys. The DNS system in turn does not handle
large record sizes well (see RFC8301), which is why Ed25519 public keys
would be preferable, but I'm not sure the ecosystem has caught up, so we
stay on the conservative side with RSA for now.
Closes: #210
Both rspamd and redis run on the same host by default, so a UNIX domain
socket is the cheapest way to facilitate that communication.
It also allows us to get rid of overly complicated IP adddress parsing
logic, that we can shift onto the user if they need it.
