warming cluster is not removed at new CDS response

[lambdai]

If the cluster "foo" is warming, and the next CDS response doesn't contains "foo",
the cluster "foo" will not be removed.

This doesn't follow the concept of wildcard subscription. Also, LDS api doesn't
act like this.

CC @htuch

[lambdai]

Although this behavior is even guarded by integration test.

envoy/test/integration/ads_integration_test.cc

Lines 544 to 575 in 0e42452

	// Send the first warming cluster.
	sendDiscoveryResponse<envoy::config::cluster::v3::Cluster>(
	Config::TypeUrl::get().Cluster, {buildCluster("warming_cluster_1")},
	{buildCluster("warming_cluster_1")}, {"cluster_0"}, "2");
	test_server_->waitForGaugeEq("cluster_manager.warming_clusters", 1);
	EXPECT_TRUE(compareDiscoveryRequest(Config::TypeUrl::get().ClusterLoadAssignment, "1",
	{"warming_cluster_1"}, {"warming_cluster_1"}, {"cluster_0"}));
	// Send the second warming cluster.
	sendDiscoveryResponse<envoy::config::cluster::v3::Cluster>(
	Config::TypeUrl::get().Cluster, {buildCluster("warming_cluster_2")},
	{buildCluster("warming_cluster_2")}, {}, "3");
	test_server_->waitForGaugeEq("cluster_manager.warming_clusters", 2);
	// We would've got a Cluster discovery request with version 2 here, had the CDS not been paused.
	EXPECT_TRUE(compareDiscoveryRequest(Config::TypeUrl::get().ClusterLoadAssignment, "1",
	{"warming_cluster_2", "warming_cluster_1"},
	{"warming_cluster_2"}, {}));
	// Finish warming the clusters.
	sendDiscoveryResponse<envoy::config::endpoint::v3::ClusterLoadAssignment>(
	Config::TypeUrl::get().ClusterLoadAssignment,
	{buildClusterLoadAssignment("warming_cluster_1"),
	buildClusterLoadAssignment("warming_cluster_2")},
	{buildClusterLoadAssignment("warming_cluster_1"),
	buildClusterLoadAssignment("warming_cluster_2")},
	{"cluster_0"}, "2");
	// Validate that clusters are warmed.
	test_server_->waitForGaugeEq("cluster_manager.warming_clusters", 0);

IMHO it's a bug. Should we create a flag to optionally introduce new behavior.

[htuch]

Seems broken, probably don't even need a runtime flag.