I'm trying to build knot-resolver with redis support.

The alternative cache backends are not supported anymore since 2.0.0.
https://gitlab.labs.nic.cz/knot/knot-resolver/blob/v2.0.0/NEWS#L14

The cache reading design in knot-resolver is (and always was) synchronous, and that can't work well with this kind of backends due to their latency.  We don't plan to resurrect these anytime soon; it's more likely that CloudFlare will open their lua module that multi-casts cache changes to other instances.

--Vladimir