Add options to have fio latency profile a device

This adds three new options:

- latency_target. This defines a specific latency target, in usec.
- latency_window. This defines the period over which fio samples.
- latency_percentile. This defines the percentage of IOs that must
  meet the criteria specified by latency_target/latency_window.

With these options set, fio will run the described workload and
vary the queue depth between 1 and iodepth= to find the best
performing spot that meets the criteria specified by the three
options.

A sample job file is also added to demonstrate how to use this.

Signed-off-by: Jens Axboe <axboe@kernel.dk>