Make recursor & dnsdist communicate (ECS) 'variable' status #7209
Rebase of #7035 + some additional fixes.
DO NOT MERGE - code works, but needs review
With this PR, when the recursor is configured to 'use-incoming-edns-subnet', it will add a /0 ECS scope to responses that are not variable.
Meanwhile with this PR, dnsdist learns that /0 answers from backends are invariant, and uses these for anyone who asks, independent of source address. The details are described more fully in this gist: https://gist.github.com/ahupowerdns/cebbddb1cee967c4c6b31176c213dcb8
This code has been tested in production and positively zooms.
…g-ecs, for the eventual benefit of dnsdist