clusterimpl: Switch Child LB to Graceful Switch

[zasweq]

Me Eric and Michael (who was guided by Mark) came to the conclusion that the Child LB of the cluster impl should be a Graceful Switch Balancer. This is needed to correctly support the scenario where the Child Type changes, and is currently a correctness issue.

Eric also mentioned writing a cross language interop test for this case.

[ejona86]

It seems the code only uses the child policy type the first time it is called. I the child type changes, this could easily cause a ClassCastException and cause the channel to panic.

grpc-java/xds/src/main/java/io/grpc/xds/ClusterImplLoadBalancer.java

Lines 117 to 136 in bbe5a02

	if (cluster == null) {
	cluster = config.cluster;
	edsServiceName = config.edsServiceName;
	childLbHelper = new ClusterImplLbHelper(
	callCounterProvider.getOrCreate(config.cluster, config.edsServiceName),
	config.lrsServerInfo);
	childLb = config.childPolicy.getProvider().newLoadBalancer(childLbHelper);
	// Assume load report server does not change throughout cluster lifetime.
	if (config.lrsServerInfo != null) {
	dropStats = xdsClient.addClusterDropStats(config.lrsServerInfo, cluster, edsServiceName);
	}
	}
	childLbHelper.updateDropPolicies(config.dropCategories);
	childLbHelper.updateMaxConcurrentRequests(config.maxConcurrentRequests);
	childLbHelper.updateSslContextProviderSupplier(config.tlsContext);
	childLb.handleResolvedAddresses(
	resolvedAddresses.toBuilder()
	.setAttributes(attributes)
	.setLoadBalancingPolicyConfig(config.childPolicy.getConfig())
	.build());