improve mismatch identifiers for RRsets

Fields such as question, answer, authority, additional can have mismatching RR sets. It's not useful to attempt to group them together like mismatches in other fields. It's also difficult to represent this mismatch with a string representation. It currently contains the entire RR set, which is difficult to handle/format/read. Mismatches should have a short and readable identifier.

Mismatches of RRsets should be handled differently, perhaps only printing qid of the mismatch. The full RRset could then be displayed with a separate tool.