Provide REST API that can be used to know whether a plan key is being deleted

Because of BAM-10174, there is no way to create a new plan using the a key that is in the process of being deleted. However, there is no way to see if a specific plan key is being removed since REST reports the plan no longer exists on a query, but you get an error on trying to do a create. Of course, an alternative for REST would be provide an option on the delete request to wait until it is really gone.