Fixed leaks & problems in RDATA after failed IXFR

- Shallow-copy of RRSets lead to broken RDATA chains in case of
  failed transfer. Replaced with deep-copy and adjusted cleanup
  accordingly.
- Fixed possible leak after successful transfer, if more RDATA of
  one RRSet were removed in the transfer.
- Fixed several leaks after failed update.

TODO: There is still a problem with node owners' nodes after
      failed transfer.

refs #1577 @4h