Fixed leaks from failed IXFR.

- Storing each RDATA separately during changeset application. The
  chains may get corrupted due to other changes and thus may not be
  deallocated properly. This caused leaks in some cases when the
  IXFR failed.
  This includes counting how many space we must reserve by counting
  number of RDATA to be stored.
- Function for getting RDATA count from the RDATA chain itself