Add support for latency probing over an interval of load

Add support for latency probing over an interval of load

Provide a way to easily run a latency probe on the device.
You define a job with peak parameters, and then probe settings
for generating iops/latency numbers based on that workload.
The latter looks something like this:

iodepth_mode=stepped:10-130/10,5/10

which has the format of:

low_percentage-high_percentage/step,ramp_time/run_time

The above would probe from 10% of peak performance to 130%,
in steps of 10%. For each step, it would run a 5 second ramp,
then do 10 seconds of testing. For percentages <= 100%,
fio will limit the IOPS. For percentages above, it'll ramp up
the queue depth. For each section run, it'll look the avg
completion latency associated with that queue depth / iops
setting.

Has normal output (which sucks), and json output. Still
experimenting, not final form yet.

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