Refactor ThreadPool code for clarity and to fix some bugs with using the pool from different threads in parallel. We have a singleton pool, and need to ensure it is created only once and used only by one thread at a time. This model is a simple way to ensure we use a number of threads equal to the number of cores, more or less (a pool per Module might lead to number of cores * number of Modules being optimized). This refactoring adds a parent pointer in the worker threads (giving them direct access to the pool makes it simpler to make sure that pool and thread creation and teardown are threadsafe). This commit also adds proper locking around pool creation and pool usage.