zone - lookup of the additionals

Rationale

As shown in the benchmark results at https://www.knot-dns.cz/pages/benchmark.html there is a performance drop in TLD cases that is not so significant in other test cases. I have sampled the process and it seems a lookup for RRs to ADDITIONALS section is the culprit. The streamlined dnames sped up most of the processing. However, in TLD case, there are usually 2 NS records in an answer and both trigger a lookup for ADDITIONAL section.

Since only a few selected types do this (those having a dname on the right side), we could store the pointers to additional rrsets in the node or rrset. Before, it was stored in the dname structure, but this is no longer possible. I think rrset, that should undergo a refactoring shortly would be an ideal candidate.