graph+autopilot: let autopilot use new graph ForEachNode method

Which passes a NodeRTx to the call-back instead of a `kvdb.RTx`.

rpcserver.go

@@ -6533,10 +6533,8 @@ func (r *rpcServer) DescribeGraph(ctx context.Context,
 	// First iterate through all the known nodes (connected or unconnected
 	// within the graph), collating their current state into the RPC
 	// response.
-	err := graph.ForEachNode(func(_ kvdb.RTx,
-		node *models.LightningNode) error {
-
-		lnNode := marshalNode(node)
+	err := graph.ForEachNode(func(nodeTx graphdb.NodeRTx) error {
+		lnNode := marshalNode(nodeTx.Node())
 
 		resp.Nodes = append(resp.Nodes, lnNode)