the cookies require libknot>=2.3, Makefile didn't
check for that, and turning cookies off wasn't
possible either

Introduced kr_rplan_push_empty() to create empty query for DNS cookies.

The control structure is part of the resolver context.

Use ENABLE_cookies=yes variable to compile functionality.

The cookies layer injects a new query into the plan when a DADCOOKIE
response is detected. After failing the second attempt a TCP fallback is
signalised.

The source address was always put into a subsequent query.

This simplifies the response source identification on the cookie module.

* in the begin() layer, the incoming query is
  exposed as req->qsource.packet, it is invalidated
  after begin() and should not be modified
* the destination address (local interface) is
  also tracked for filtering purposes

during the consume step, the information about
upstream authoritative (address and current rtt)
is exposed in the request structure, just like
information about current query