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/pathfind_test.go

@@ -165,12 +165,13 @@ func parseTestGraph(path string) (*channeldb.ChannelGraph, func(), aliasMap, err
 		}
 
 		dbNode := &channeldb.LightningNode{
-			AuthSig:    testSig,
-			LastUpdate: time.Now(),
-			Addresses:  testAddrs,
-			PubKey:     pub,
-			Alias:      node.Alias,
-			Features:   testFeatures,
+			HaveNodeAnnouncement: true,
+			AuthSig:              testSig,
+			LastUpdate:           time.Now(),
+			Addresses:            testAddrs,
+			PubKey:               pub,
+			Alias:                node.Alias,
+			Features:             testFeatures,
 		}
 
 		// We require all aliases within the graph to be unique for our