Cleanup AbstractGeoDnsService.addTargetHost

- avoid NPE when hostname==null and ip!=null, and !USE_HOSTNAMES
- improve logging, so if log about hostname missing and then subsequent
  can't infer geo, then still log about the latter.
- general code cleanup; but more could be done.
Merge pull request #966 from aledsage/fix/AbstractGeoDnsService-addTarget

Cleanup AbstractGeoDnsService.addTargetHost