message: avoid magic number in AsyncService
diff: @@ -147,7 +147,9 @@ MasterService::Stub::Stub(
  }
  MasterService::AsyncService::AsyncService() {
  - for (int i = 0; i < 10; ++i) {
  + int method_len = sizeof(grpcMasterService_method_names) /
  + sizeof(grpcMasterService_method_names[0]);
  + for (int i = 0; i < method_len; ++i) {
  AddMethod(new ::grpc::internal::RpcServiceMethod(
  grpcMasterService_method_names[i],
  ::grpc::internal::RpcMethod::NORMAL_RPC, nullptr));
  