kr_ranked_rrarray*: avoid duplicate RRsets on wire

Fixes https://gitlab.labs.nic.cz/knot/resolver/issues/198. We can't let multiple "matching RRsets" to the wire, and we can't just merge the sets from multiple queries either. The only way is to choose either of the sets and put it on the wire. ATM the last one wins.

Common ocurrence of the bug: if www.example.cz was a CNAME for example.cz and we ask for a non-existent type, we would get the SOA record twice in the final answer.

A few related changes:

• don't just assert, also return error code if -DNDEBUG
• kr_ranked_rrarray_set_wire: don't do full-content comparison anymore; see the first paragraph in this commit message for the reasons
• minor refactoring of that code, more comments, etc.

marked as a Work In Progress from cca563c1f7e811e59a77a06c1ed33176a0282778

I wonder if there is something which prevents people from sending multiple distincts RRsets in one answer. I mean: What RFC says that IN A 127.0.0.1 and IN A 192.0.2.1 must be always in a single RRset?

I can see that things get complicated when we talk about signing, just thinking aloud...

@pspacek: RRs from the same answer do get merged by ranked_rrarray_add(). On wire each RR is "on its own" anyway, and we merge them even if the "set" isn't consecutive.

The point here is that ranked_rr_array_t contains RRs from multiple packets, and it's forbidden to create such combinations.

resolved all discussions

unmarked as a Work In Progress

changed the description

added 3 commits

• 58b7af80...4fa31008 - 2 commits from branch master
• 8d79b517 - kr_ranked_rrarray*: avoid duplicate RRsets on wire

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mentioned in commit c41d3c26

merged