routing: Require adding edge to node before adding node.

This commit introduces the requirement specified in BOLT#7,
where we ignore any node announcements for a specific node
if we yet haven't seen any channel announcements where this
node takes part. This is to prevent someone DoS-ing the
network with cheap node announcements. In the router this
is enforced by requiring a call to AddNode(node_id) to
be preceded by an AddEdge(edge_id) call, where node_id is
one of the nodes in edge_id.

routing/errors.go

@@ -35,7 +35,8 @@ const (
 	ErrOutdated
 
 	// ErrIgnored is returned when the update have been ignored because
-	// this update can't bring us something new.
+	// this update can't bring us something new, or because a node
+	// announcement was given for node not found in any channel.
 	ErrIgnored
 )