Implement and use a hierarchy stack to avoid storing delete events for child entities

• Type: Technical Task
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: 6.2.0 CE M6
• Fix Version/s: 6.2.0 CE B1
• Component/s: Activities widget, Staging > Export/Import, ~ [Archived] Social Networking
• Labels: None
• Git Pull Request: https://github.com/mhan810/liferay-portal/pull/147

When a container entity is being deleted, it recursively deletes all of its children. Having a separate entry for each of these delete events is unnecessary, so a hierarchy stack should be used so that child events can be detected by the system event service.

For delete events the one for the parent should be enough (deleting the parent entity on the import side should delete all child entities).

To handle this filtering generically, we should have a place (portal.properties for now) to configure this behavior for events.