Attaches the current
thread to the pool.
This function attaches the current thread to the pool so that it may be
used for executing submitted function objects. Blocks the calling thread
until the pool is stopped or joined and has no outstanding work.
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)