resolved: when synthesizing stub replies from multiple upstream packet, let's avoid...

resolved: when synthesizing stub replies from multiple upstream packet, let's avoid RR duplicates

If we synthesize a stub reply from multiple upstream packet (i.e. a
series of CNAME/DNAME redirects), it might happen that we add the same
RR to a different reply section at a different CNAME/DNAME redirect
chain element. Let's clean this up once we are about to send the reply
message to the client: let's remove sections from "lower-priority"
sections when they are already listed in a "higher-priority" section.