autopilot: convert Peer to route.Vertex

In preparation for removing the ForEachNode/ForEachChannel call pattern,
we refactor the autopilot subserver by simplifying the `ChannelEdge`
such that it no longer returns the `Node` type which has a
`ForEachChannel` method on it. The aim is to completely remove the
`ForEachChannel` usage.

autopilot/interface.go

@@ -72,7 +72,7 @@ type ChannelEdge struct {
 
 	// Peer is the peer that this channel creates an edge to in the channel
 	// graph.
-	Peer Node
+	Peer route.Vertex
 }
 
 // ChannelGraph in an interface that represents a traversable channel graph.