Fix for threadpool abort function

Fixed an issue where if threadpool was aborted when some threads were in a running state the threadpool would never exit. This was due to the fact that the thread terminate task (func == NULL && args == NULL) could have been pushed to queues which were inactive. This change adds a uint64_t mask of threads that have already exited so whenever the terminate task is sent to another queue, it is ensured that the queue used will wake up at least one tpool worker.

Also added pushTpool_mask which allows to specify a subset of threads to push the task to but its only used internally.

Also added in this commit: A way to allocate thread-safe storage for thread pool workers via get_tpool_worker_index. This index is unique to the thread pool worker within one threadpool and can be used to access arrays in a thread-safe manner.

An example of such use was added in a testcase that was added which compares delay between actors and threadpool workers.