AIO operations now add timers (ticket #2162). Each AIO (thread IO) operation being run is now accompanied with 1-minute timer. This timer prevents unexpected shutdown of the worker process while an AIO operation is running, and logs an alert if the operation is running for too long. This fixes "open socket left" alerts during worker processes shutdown due to pending AIO (or thread IO) operations while corresponding requests have no timers. In particular, such errors were observed while reading cache headers (ticket #2162), and with worker_shutdown_timeout.