Add support for non-uniformly random file service type

Similar options to random_distribution, this is just for the
selection of files. Like that option, you can do:

file_service_type=zipf:1.2

and get files selected through a zipfian distribution. This extends
the concept of hot and cold IO regions to have hotter and colder files
as well.

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