Add the file sharing bits

Add the file sharing bits

When you use the same filename for several jobs now, they will share
the same file structure. Enable locking through two new options:

- lockfile. If set, a semaphore is associated with the file and it is
  held from ->prep() to ->queue() has done its work.
- lockfile_batch. This controls how many IOs the job gets to do per
  semaphore acqusition.

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