Implement saner smoke test for connectivity

[gmega]

Both Codex app and the CLI tool @hackyguru is working on are displaying DHT routing peers as a check that you're connected to the network, showing those as "connected peers". This is inaccurate as the DHT provides very weak guarantees in that sense - peers might be stale or dead. We should think of a slightly better way of doing this. Some low-cost options:

1. show libp2p peers. Would be great, but when you bootstrap you have $0$ of those so may not be that helpful;
2. show DHT peers for which we have recent liveness checks. Guarantees that at least you were able to exchange a ping with the peer in recent times, meaning you can reach a Codex peer and it can reply.