Allow for saving verify state on normal/timeout exit from a job

Signed-off-by: Jens Axboe <axboe@fb.com>
Add verify_state_save option

Then we can control this independently of the verify trigger. This
is useful for, eg, setting a timeout or amount of IO to be written,
then verify to that point later on.

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