Function set_async

extern/boost/boost/thread/future.hpp:2036–2039

todo this function must be private and friendship provided to the internal users.

            // todo this function must be private and friendship provided to the internal users.
            void set_async()
            {
              this->future_->set_async();
            }
            // todo this function must be private and friendship provided to the internal users.
            void set_deferred()
            {