AIO operations now add timers (ticket #2162).

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.