[Rendezvous] How exactly does Unregister work?

[thomaseizinger]

The spec is fairly thin on the behaviour of Unregister.

I am opening this issue to discuss primarily two aspects:

1. According to af5ba41, the id in the Unregister message is the PeerId. Why do we need to send this? We know who is sending the message because our connection is authenticated.
2. Is there a response to an Unregister message? I don't think there needs to be because there is no useful information the server can share with the client.

Tagging interest group + author: @daviddias, @whyrusleeping, @Stebalien, @jacobheun, @yusefnapora, @vasco-santos, @vyzo (Please let me know if one shouldn't do this)