fio: add job_runtime to the thread json output

fio: add job_runtime to the thread json output

I discovered that the 'elapsed' value of each job is the overall time
the whole fio run took, not the per-job elapsed time, despite it being
in the per-job section of the json output.  Rather than break everybody
by fixing this, just add a 'job_runtime' value to the job section and
include the time the thread ran in milliseconds.  This is handy for jobs
that want to compare runtime of two threads in different cgroups.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>