[boost] [defer/thread_pool] - added again to boost vault

Hello, because on the user list a library which allows reuse of threads was requested I've added the defer library, originally developed by Simon Meiklejohn, to the boost vault (concurrent programming - I don't know why it was removed?). I've modified the library a little bit - a threaded queue for the thread pool was added. Maybe we can discuss the solution?! regards, Oliver