This patch makes two absolutely critical changes:
  - If an ed25519 identity is not as expected when creating a channel,
    we call that channel unsuccessful and close it.
  - When a client creating a channel or an extend cell for a circuit, we
    only include the ed25519 identity if we believe that the node on
    the other side supports ed25519 link authentication (from
    #15055).  Otherwise we will insist on nodes without the right
    link protocol authenticating themselves.
  - When deciding to extend to another relay, we only upgrade the
    extend to extend by ed25519 ID when we know the ed25519 ID _and_
    we know that the other side can authenticate.

This patch also tells directory servers, when probing nodes, to
try to check their ed25519 identities too (if they can authenticate
by ed25519 identity).

Also, handle the case where we connect by RSA Id, and learn the
ED25519 ID for the node in doing so.