aio: Don't silently drop wait_event_info

Hi,

While working on something else in the AIO code I noticed that
FileStartReadV() accepts a wait_event_info argument but never passes
it. Instead pgaio_io_perform_synchronously() hardcodes the wait event
to WAIT_EVENT_DATA_FILE_READ, so a wait event supplied by the caller
is silently ignored.

This is not a live bug today. The only caller of FileStartReadV() is
md.c, and it passes exactly the WAIT_EVENT_DATA_FILE_READ that is
hardcoded anyway; pgaio_io_start_writev() has no callers at all yet.

Fix this by storing the wait event in PgAioOpData and reporting it.

--
Regards,
Nazir Bilal Yavuz
Microsoft