Serve basic HTTP response on the listen address

[sisou]

The PoW client served a basic, one-line HTTP 200 OK response on the listen address, and handled Websockets on the /ws path I think. See https://node.v2.test.nimiqwatch.com.

I don't know if it's possible to serve both WebSocket and HTTP response on the same path, and I think the PoS client is listening on the / root path, but could we maybe add another path to check for public availability?

The main reason for this feature request is that many validtors-to-be are struggling with the AutoNAT warning that their node is not publicly reachable and validator functionality won't be working (which is false in itself in my experience, it work's just fine even if you can only connect outbound).

So if we serve a basic HTTP response from the node, we don't have to muck around with telnet, wscat or a custom seed node config to test their public reachability.

[hrxi]

Sounds useful to me.