Optimize DeleteObjectHierarchyWorker to memoize objects it loads from database so that not all hierarchy entries handling would require obtaining the parent objects from the database.

Additionally it is possible to get the delete_method from the association itself. We may use this to automatically detect :delete and :delete_all associations, reduce configuration complexity and be able to employ :delete_all to save possibly loads of queries for individual objects.

The worker I'm talking about is as implemented with https://github.com/3scale/porta/pull/3958