Provide starttime_monotonic

Add the field to the flow reports, in addition to starttime, lasttime
and lasttime_monotonic.

We want to be able to compute a guaranteed non-negative flow duration.
1 job for starttime_monotonic in 1 minute and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #5557
test

00:01:14