Fixed broken DDNS processing, refactored related functions.

Fixed:
- Memory errors when processing RR sets
- Improper use of response size
- Repetitive code
- Wrong return codes
- Bad created changesets (no SOA)
- Improper freeing of changesets
- Improper use of changesets
- UPDATE of RRs where name is equal to updated zone
- Removed RRSet RDATA wasn't deleted
- Leaks

refs #937