graph/db: split HasNode into two methods

HasNode currently is very v1 specific since it returns a time.Time
timestamp which is specific to V1 node announcements. However, it is
mostly only used for the "exists" return value. So here we split it up
into HasNode which just checks existence and HasV1Node which retains the
same behavaiour as before.

graph/builder.go

@@ -874,7 +874,7 @@ func (b *Builder) assertNodeAnnFreshness(ctx context.Context, node route.Vertex,
 	// node announcements, we will ignore such nodes. If we do know about
 	// this node, check that this update brings info newer than what we
 	// already have.
-	lastUpdate, exists, err := b.cfg.Graph.HasNode(ctx, node)
+	lastUpdate, exists, err := b.cfg.Graph.HasV1Node(ctx, node)
 	if err != nil {
 		return fmt.Errorf("unable to query for the "+
 			"existence of node: %w", err)