Try handling the case when the first side knows the ecdsa key of
the second, but the second not the key of the first.
(And both have the experimental protocol enabled)
Better log messages when we already know the peer's key during an upgrade.

If the peer presents a different one from the one we already know, log
an error. Otherwise, log an informational message, and terminate in the
same way as we would if we didn't already have that key.