Add max execution time to cronjob

[zachomedia]

This will prevent a stack of cronjobs running if something causes the cronjob to fail or hang.

[sylus]

Think this will work?

All newer jobs will replace the existing one if is hanging, also check that at least runs and we can enforce a max of 5 so don't get inundatedd with calls.

The core-cron issue itself should be fixed so think this is reasonable?