TaskManager waits for all threads in the ThreadPool

[aleks-f]

TaskManager uses either the global or user-supplied external ThreadPool. TaskManager::joinAll() calls the ThreadPool::joinAll(), which means it will have to wait for all threads, including the ones not running the managed tasks, to complete execution. https://github.com/pocoproject/poco/blob/92fac970d4b6161fd58335bfe1492689ba5a80a3/Foundation/src/TaskManager.cpp#L77-L80