Fix verify state for multiple files

Fix verify state for multiple files

Fio currently only logs the completions on a per-job basis, but
that breaks down if the job uses multiple files and we rely
on the verify state save/load to check when to stop verifying.

This patch extends the verify state format to be version 3, and
removes support for v1/v2. The new version has support for
multiple files.

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