Leases all available cron jobs.

def LeaseCronJobs(self, cronjob_ids=None, lease_time=None):
    """Leases all available cron jobs."""
    leased_jobs = []

    now = rdfvalue.RDFDatetime.Now()
    expiration_time = now + lease_time

    for job in itervalues(self.cronjobs):
      if cronjob_ids and job.cron_job_id not in cronjob_ids:
        continue
      existing_lease = self.cronjob_leases.get(job.cron_job_id)
      if existing_lease is None or existing_lease[0] < now:
        self.cronjob_leases[job.cron_job_id] = (expiration_time,
                                                utils.ProcessIdString())
        job = job.Copy()
        job.leased_until, job.leased_by = self.cronjob_leases[job.cron_job_id]
        leased_jobs.append(job)

    return leased_jobs