balancer: stop using picker from old balancer with switching balancers (#2833)
balancer: stop using picker from old balancer with switching balancers (#2833)


menghanl/grpc-go,grpc/grpc-go,dfawley/grpc-go,menghanl/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go