[boost] [pool] Conversion to Boost.Thread for singleton_pool

Boost.Pool contains a private mutex implementation used as the default mutex type for singleton_pool (which provides a thread-safe API). Now that Boost.Thread is well-established and in line with TR1, I'd like to jettison the private mutex implementation inside Boost.Pool and switch the default mutex type to be boost::mutex. The obvious consequence of this change is that singleton_pool becomes dependent on Boost.Thread. The dependency would exist only for singleton_pool and its clients and would not affect the other Boost.Pool interfaces. Anyone have any objections or reasons why this wouldn't be a good idea? -Chris