This is now permitted in the offers PR, so we should support it. But
we can't just look up in the gossmap, since the "short_channel_id"
could be an alias. So we get lightningd to tell us all scid->peer
mappings, and look up in that.
Changelog-Added: Protocol: onion messages can now be forwarded by short_channel_id.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Allows for caller to log, but more importantly, when we add a command to
inject onion messages, allows for us to capture the error.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Instead of open coding in connectd/onion_message, we move it to common
with a nice API.
This lets us process the BOLT test vectors.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
