Need mechanism for flushing old lights

Once a light (name) is no longer used, the light will continue to exist, cluttering up the data store and the admin panel. There is no way to differentiate between unused stoplights and stoplights which have not been run recently. On the other hand, the cost associated with deleting a light which is active (so long as it doesn't have recorded failures) is almost nil.

I propose we implement a method which deletes all lights without failures, to be invoked from the admin panel.

The one caveat which springs to mind is that defunct stoplights with failures will never be cleaned up.