message: Add dispatch::make_ready_future(void)
diff: @@ -520,6 +520,11 @@ auto make_ready_future(T&& value) -> future<T>
  return boost::make_ready_future(std::forward<T>(value));
  }
  +inline future<void> make_ready_future()
  +{
  + return boost::make_ready_future();
  +}
  +
  template<typename T>
  auto make_exceptional_future_from_current() -> future<T>
  {
  