graph/db: let ForEachNodeCached take a context

rpcserver.go

@@ -7130,7 +7130,7 @@ func (r *rpcServer) GetNetworkInfo(ctx context.Context,
 	// network, tallying up the total number of nodes, and also gathering
 	// each node so we can measure the graph diameter and degree stats
 	// below.
-	err := graph.ForEachNodeCached(func(node route.Vertex,
+	err := graph.ForEachNodeCached(ctx, func(node route.Vertex,
 		edges map[uint64]*graphdb.DirectedChannel) error {
 
 		// Increment the total number of nodes with each iteration.