the thread pool that it has some outstanding work to do.
void on_work_started() const;
This function is used to inform the thread pool that some work has begun.
This ensures that the thread pool's join() function will not
return while the work is underway.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)