Add some support for a verify backlog

Add some support for a verify backlog

Instead of writing everything and then verifying everything,
allow a job to specify incremental verify stages. This can
help reduce memory consumption of fio, since we don't have
to store a huge number of verify entries to be processed
when the write phase is complete.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>