[boost] [fiber] design fault with multithreading

The current design doesn't allow a work-stealing algorithm to distribute work perfectly. Unfortunately, the use of sleep queue and remote ready queue made it insufficient for schedulers like shared_work. Instead here, I'd like to propose it as non-pure virtual function in the algorithm class. This allows schedulers to override the queues, and enables the use of underlying implementation's timer manager, which is included in Asio. Friendly notice: please use "Reply All" when replying to me.