Currently it deadlocks during process termination, when
atexit first calls handler.close() and then logging.shutdown(),
that in turn calls handler.flush() without arguments.

handler.close() kills the worker, and then handler.flush() forever waits for the dead worker to send the messages from the queue.

After this change, the deadlock is still possible if something concurrently closes the handler from another thread during the flush. However, this scenario is much less likely.