Fix client connecting via gossip seeds

- This fixes the issue reported on the list in the "connection to cluster using gossip seeds fails (client api)" thread.
- ClusterSettingsBuilder now forces you to choose first between DNS or Gossip Seed configuration and then presents only the correct configuration points.
- This is a breaking API change, however I think it's worth putting into the 3.0.0 release as the behaviour at the moment is completely broken.