Fixed IXFR replies when no data are available.

- Function for comprehensive comparing serials added to nameserver.
- Added function for loading serials from IXFR query.
- Loading serials before loading changesets in xfr-handler.
- Sending FORMERROR in proper cases after loading changesets
  fails.
- No data in journal results in fallback to AXFR.
- zones_xfr_load_changesets() now leaves changesets empty if the
  slave's serial is equal or larger than the master's serial.
  This results in the IXFR sending function to send out response
  with single SOA.

fixes #1409 @2h