xds/cdsbalancer: move xds client close to run() (#4273)
xds/cdsbalancer: move xds client close to run() (#4273)

Otherwise client may be used by run() after closed.
grpc/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go,grpc/grpc-go