graph/db: gracefully handle duplicate node announcements

It can happen that we are handling 2 of the same node announcements in
the same batch transaction. In that case, our `UpsertNode` conflict
assertion may fail. We need to handle this gracefully.

graph/db/graph_test.go

@@ -125,6 +125,14 @@ func TestNodeInsertionAndDeletion(t *testing.T) {
 	require.NoError(t, graph.AddNode(ctx, node))
 	assertNodeInCache(t, graph, node, testFeatures)
 
+	// Our AddNode implementation uses the batcher meaning that it is
+	// possible that two updates for the same node announcement may be
+	// processed in the same batch. So to avoid the conflict error (since we
+	// require at the DB level that the new timestamp is strictly
+	// greater than the previous one), we need to gracefully handle the
+	// case where the exact same node announcement is added twice.
+	require.NoError(t, graph.AddNode(ctx, node))
+
 	// Next, fetch the node from the database to ensure everything was
 	// serialized properly.
 	dbNode, err := graph.FetchNode(ctx, testPub)