Leases all available cron jobs.

def LeaseCronJobs(self, cronjob_ids=None, lease_time=None, cursor=None):
    """Leases all available cron jobs."""
    now = rdfvalue.RDFDatetime.Now()
    now_str = mysql_utils.RDFDatetimeToTimestamp(now)
    expiry_str = mysql_utils.RDFDatetimeToTimestamp(now + lease_time)
    id_str = utils.ProcessIdString()

    query = ("UPDATE cron_jobs "
             "SET leased_until=FROM_UNIXTIME(%s), leased_by=%s "
             "WHERE (leased_until IS NULL OR leased_until < FROM_UNIXTIME(%s))")
    args = [expiry_str, id_str, now_str]

    if cronjob_ids:
      query += " AND job_id in (%s)" % ", ".join(["%s"] * len(cronjob_ids))
      args += cronjob_ids

    updated = cursor.execute(query, args)

    if updated == 0:
      return []

    cursor.execute(
        "SELECT job, UNIX_TIMESTAMP(create_time), enabled,"
        "forced_run_requested, last_run_status, UNIX_TIMESTAMP(last_run_time), "
        "current_run_id, state, UNIX_TIMESTAMP(leased_until), leased_by "
        "FROM cron_jobs "
        "FORCE INDEX (cron_jobs_by_lease) "
        "WHERE leased_until=FROM_UNIXTIME(%s) AND leased_by=%s",
        [expiry_str, id_str])
    return [self._CronJobFromRow(row) for row in cursor.fetchall()]