Breaking changes in peer message protocol should be discovered as soon as possible

[contrun]

The error here #231 is actually caused by a breaking change in the peer message protocol. This is because we changed the fields of the NodeAnnouncement message. This breaking change goes undetected. Maybe we should add some test fixtures to assure that old messages are still correctly de-serialized and processed. In this specific case, we can save the old NodeAnnouncement and try to process it with newer code in a unit test.