Record job start time to fix time pain points

Record job start time to fix time pain points

Add a new key in the json per-job output, job_start, that records the
job start time obtained via a call to clock_gettime using the clock_id
specified by the new job_start_clock_id option. This allows times of fio
jobs and log entries to be compared/ordered against each other and
against other system events recorded against the same clock_id.

Add a note to the documentation for group_reporting about how there are
several per-job values for which only the first job's value is recorded
in the json output format when group_reporting is enabled.

Fixes #1544

Signed-off-by: Nick Neumann nick@pcpartpicker.com