Don't blacklist unresolvable hubs

This fixes #271.

Revert this blacklisting policy. We should keep trying so that gateways that have lost their wireless connection can recover the hub when it comes back in discovery range. Main issue is that avahi hangs on to a `ghost` advertisement of the hub.

@piyushk is this the right way we should handle this? Do you have a better idea?

There are some other minor things in this pull request as well - in the comments.