Stores a cron job run object in the database.

def WriteCronJobRun(self, run_object):
    """Stores a cron job run object in the database."""
    if run_object.cron_job_id not in self.cronjobs:
      raise db.UnknownCronJobError("Job with id %s not found." %
                                   run_object.cron_job_id)

    clone = run_object.Copy()
    clone.timestamp = rdfvalue.RDFDatetime.Now()
    self.cronjob_runs[(clone.cron_job_id, clone.run_id)] = clone