RRSet merging improvements.

- In IXFR/AXFR processing replaced rrset_merge() by
  rrset_merge_no_dupl().
- In some other cases it is not reuqired.
- Modified rrset_compare() function to not check TTL
  TODO: check if this doesn't cause problems elsewehere.
- Needs some testing.

refs #1953 @20m