In particular, these functions are the ones that set the identity of
a given connection or channel, and/or confirm that we have learned
said IDs.

There's a lot of stub code here: we don't actually need to use the
new keys till we start looking up connections/channels by Ed25519
IDs.  Still, we want to start passing the Ed25519 IDs in now, so it
makes sense to add these stubs as part of 15055.