Clean up orphaned eventinstances automatically

When you delete an eventseries, the eventinstances will also be deleted.
Or - atleast that is supposed to happen. Sometimes, if there is a massive amount of instances, or if PHP gets interrupted for another reason, you might end up with deleting the series, whilst the instances still hang around.

This causes all kind of interesting problems, if you load eventinstances in APIs and what-not.

I can see the module supplies a form, for deleting all the orphaned events at admin/structure/events/orphaned-instances

But, I feel like this should not be happening manually.
Instead, I'd prefer to make it an option to happen automatically, as part of cron.

I've created a patch, that creates a setting for this.