xds: use order-deterministic locality map for preserving most recent locality update to eliminate test flakiness (#5921)
xds: use order-deterministic locality map for preserving most recent locality update to eliminate test flakiness (#5921)

* xds: use order-deterministic locality map for preserving most recent locality update to eliminate test flakiness

* make localityMap immutable

grpc/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java