Add support for queuing > 1 command at the time

For the async engines, we currently do queuing by issuing one
command at the the time. Improve this by adding a ->commit()
hook to complement the ->queue() hook. When ->queue() returns
FIO_Q_BUSY, call ->commit() to actually send off the io to the
kernel.

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