Inform the thread pool that some
work is no longer outstanding.
This function is used to inform the thread pool that some work has finished.
Once the count of unfinished work reaches zero, the thread pool's join()
function is permitted to exit.
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)