cron_debug.php script broken, calls some non existing cronjob functions.

All cronjobs (or the base 'cronjob' class') need an isRunning() function added or they can't be run manually:

PHP Fatal error:  Uncaught Error: Call to undefined method cronjob_monitor_email_quota::isRunning() in /usr/local/ispconfig/server/cron_debug.php:76
Stack trace:
#0 {main}
  thrown in /usr/local/ispconfig/server/cron_debug.php on line 76

Cf. b34b1ad5