Misuse of io_getevents() causes wake-ups at least twice per second

In the asynchronous I/O interface, InnoDB is invoking io_getevents() with a timeout value of half a second, and requesting exactly 1 event at a time.

Apparently, the reason to have such a short timeout is to facilitate shutdown.

We can do better: Use an infinite timeout, wait for a larger maximum number of events. For shutdown, submit a dummy request that informs the io_getevents() caller thread to terminate.