Add low water mark for queuing depth

Add low water mark for queuing depth

Current fio will attempt to keep the queue full at all times,
but sometimes that's not what you want. Add iodepth_low to
indicate a low water mark for queuing depth, so that when we
see a FIO_Q_BUSY or run out of free requests, let the queue
drain down to the iodepth_low setting before building it up
again.

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