Under certain conditions, when splicing a new channel quickly enough, an old channel announcement would emit *after* `mutual_splice_lock` and *before* announcement signature exchange.
Since the original channeld wouldn’t start the announcement timer until signatures were exchagned, this wasn’t an issue before.
Now splicing enables us to go from having announcement sigs to losing them, so we have to be prepared for this case.
Changelog-None
5aea5fff2f