Treat discovery peer with unknown endpoint as unknown

[mbaxter]

PR description

Update discovery logic to consider a peer with an unknown discovery endpoint to be unknown regardless of whether we've encountered a peer with the same node id before. This makes the discovery logic more forgiving in the face of node restarts.

If nodeA bonds with nodeB, then nodeB crashes and restarts with a different ip address or listening port, nodeA would previously continue trying to communicate with nodeB at its original address. With these changes, nodeA will now treat the resurrected nodeB as a new peer and communicate with it on its updated endpoint.

Opening this as a draft PR to get input on whether people think this is a good idea.