Allow one-sided upgrades to Ed25519.

Allow one-sided upgrades to Ed25519.

This deals with the case where one node knows the Ed25519 key of another node, but not the other
way around. This was blocked by an overly paranoid check in id_h(). The upgrade_h() function already
handled this case, and the node that already knows the other's Ed25519 key checks that it has not
been changed, otherwise the connection will be aborted.