graph/db: let ForEachChannel take a context

rpcserver.go

@@ -6730,7 +6730,7 @@ func (r *rpcServer) DescribeGraph(ctx context.Context,
 	// Next, for each active channel we know of within the graph, create a
 	// similar response which details both the edge information as well as
 	// the routing policies of th nodes connecting the two edges.
-	err = graph.ForEachChannel(func(edgeInfo *models.ChannelEdgeInfo,
+	err = graph.ForEachChannel(ctx, func(edgeInfo *models.ChannelEdgeInfo,
 		c1, c2 *models.ChannelEdgePolicy) error {
 
 		// Do not include unannounced channels unless specifically