Fix for socket leak with "aio sendfile" and "limit_rate".

Fix for socket leak with "aio sendfile" and "limit_rate".

Second aio post happened when timer set by limit_rate expired while we have
aio request in flight, resulting in "second aio post" alert and socket leak.

The patch adds actual protection from aio calls with r->aio already set to
aio sendfile code in ngx_http_copy_filter().  This should fix other cases
as well, e.g. when sending buffered to disk upstream replies while still
talking to upstream.

The ngx_http_writer() is also fixed to handle the above case (though it's
mostly optimization now).

Reported by Oleksandr V. Typlyns'kyi.