Function SlowdownABit

cpp/src/arrow/testing/async_test_util.h:48–52

template <typename T>
AsyncGenerator<T> SlowdownABit(AsyncGenerator<T> source) {
  return MakeMappedGenerator(std::move(source), [](const T& res) {
    return SleepABitAsync().Then([res]() { return res; });
  });
}

template <typename T>
class TrackingGenerator {