- some trolls managed to flood gserver with useless URLs. They can be blocked
  by domain blocking them, but still it floods gserver table with dead entries
- this hack tries to change that so they won't enter gserver at all. Let's hope
  these trolls as `activitypub-trolls.cf` learn a lesson (and get adults soon)

Signed-off-by: Roland Häder <roland@mxchange.org>

- just for consistency ...

- moved if() block to suggested position by MrPetovan, for me I want to have all
  conditions checked at the start of the method, e.g. no unwanted null
  references or (in this case) if the URL is blacklisted
- normalized URLs are without SSL, means http://host/path/file.ext so they exist
  only once for contacts and servers (aka. instances)
- documented returned type `void`