Change IO engine queuing

Instead of always pretending to be async, let the IO engines
return FIO_Q_COMPLETED or FIO_Q_QUEUED to signal async or
sync completions regardless of their nature. This cleans up
the queuing model quite a bit.

Also fixed a verification error spotted while doing this
transformation.

The main intent of this is to allow queuing more than 1 piece
of IO at the time, that will come in a later changeset.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>