Thread pool

TLA+/PlusCal model for thread pool (runtime/thread_pool.h) is placed in thread_pool/thread_pool.tla. This model was verified in TLA+ Toolbox tool with the following parameters: max_producers = 2, max_workers = 2, queue_max_size = 2, max_controllers = 2, deadlock check. The verification process took about 2 hours on machine with the following configuration: Intel Core i7-9850H CPU @ 2.60GHz, 12 cores, 16 GB RAM.