ddm protocol version negotiation

[rcgoodfellow]

Things currently work between v1 and v2 peers because ddm v1 does no version checking at all. So ddm v2 speakers can send out discovery packets with the version set to 2, and ddm v1 speakers can send out discovery packets with the version set to 1, and as long a v2 router speaks version 1 after discovering a v1 peer, things will work. However, this will not work for version 3. So we need to implement version negotiation. This would also not work for changes in the discovery protocol, if we were to have changes there. So we need to come up with a general way for both protocols to evolve.