elastic_aio doesn't work on OS X

[grencez]

Last week I wrote a poll()-based version of elastic instead of fixing the async version on OS X, but we should use the async version if possible.

Why? AIO seems to (1) be faster than polling, (2) match the Windows IOCP approach, and (3) admit an abstraction for threads to exchange data without copying too much.

[grencez]

aio_read() on stdin fails and sets errno to EAGAIN (resource temporarily unavailable).

The same code works for regular files, so it appears that OS X just doesn't support AIO on pipes. Drat!

[grencez]

3579855 is the result of this work. No fix, but some improvements.