Finish transition to DDM protocol version 3

[rcgoodfellow]

We have implemented protocol version 3, but are still on version 2

maghemite/ddm/src/discovery/runtime.rs

Lines 50 to 66 in 6542c37

	impl DiscoveryPacket {
	fn new_solicitation(hostname: String, kind: RouterKind) -> Self {
	Self {
	version: Version::V2 as u8,
	flags: SOLICIT,
	hostname,
	kind,
	}
	}
	fn new_advertisement(hostname: String, kind: RouterKind) -> Self {
	Self {
	version: Version::V2 as u8,
	flags: ADVERTISE,
	hostname,
	kind,
	}
	}

When we make the switch, we need to consider the concerns in

• ddm protocol version negotiation #157
• test for ddm compatibility with the most recent release #275

In particular our backwards compatibility tests no longer exist. The were very messy, served no purpose when v1 ceased to exist, and it was decided that re-introducing backwards compatibility tests should have a fresh start on a second attempt at the dawn ov v3. So now here we are.