this takes more stuff out of cmd/recursor that could be used by other servers. While here reject even more qtypes from recursive resolution

So we eturn SERVFAIL correctly to clients, the same way that Cloudflare, quad9 et al. do.

With an explanation of why we lookup before we insert into the cache

that is the simplest operation, not Exchange (anymore)!

Pretty easy fix: just keep track of where we're up to each time we call resolve(). Fixes: https://todo.sr.ht/~otl/dns/4

Now we only ever send queries to nameservers, not just things we happen to find in the additional section of a reply. This change also does caching more nicely; cache any records that we get from nameservers. Authoritative answers get priority. Fixes: https://todo.sr.ht/~otl/dns/4

This is what quad9 and google's dns resolver do

it's too specific to the implementation of a recursive resolver. More stuff will be added there later I reckon.

Caller can handle the error appropriately I reckon...

That made it easy to then let us ResponseWriter to implement io.Writer

They seem generic enough to be in the package rather than in the server implementation - ignoring the cache, of course!