added some checks to lumiera_threadpool_release_thread(), maybe too much
modified thread_loop() to return a known value - 0
added checks to lumiera_thread_new()
added a check to lumiera_thread_destroy()
made lumiera_thread_destroy() unlink the thread from a pool list
updated test case to match new debug messages
added locking and checks to lumiera_threadpool_acquire_thread()
added a nobug flag to threads.c
added lumiera_thread_destroy()
added lumiera_thread_delete()