Peering

This PR address #421 (closed). I'm not certain was the best approach is. Here, the gateway will simply stop automatically removing peers when it drops below 3 (they can still be manually removed, though perhaps this should cause a warning). Another way would be to not throw any peers away, but just keep them in a "bad peers" map. Then if we fall below 3 peers, we add the best of the bad peers. Finally, if we fall below 3 peers, we could query the remaining ones for their peers. But this won't help if those peers aren't well-connected themselves.

btw, we should be using SkipNow instead of Skip. Skip produces a log entry, while SkipNow does not.