Investigate strategies for removing cascading entities

[skabashnyuk]

The problem:
User removed, but related entities like Workspaces, Factories or Permissions are not removed which leaves the system in inconsistent state and causes different problems(e.g. if workspace is not removed than namespace is still aquired by non-existing user, while it is impossible to use that namespace).

Requirements:

1. Entity related data must be removed along with the entity
2. If it is impossible to remove some of entity related data, then entity mustn't be removed either and removed data must be restored. In this way we can create application related constraints which will enforce user to perform some actions before removing the entity.
3. Sometimes it is necessary to know that entity is removed, the mechanism of notifying clients based on the EntityRemoved events which usually published after entity is removed.
   So the strategy of cascade removing should be rather on application-level than on the DBMS level which allows to control EntityRemoved events.

What should be done:
As a result we should investigate the best way for performing cascade remove which fits our application architecture with all the pros and cons of it.

Consider using:

• EventBus
• Jpa Events

[TylerJewell]

@skabashnyuk - I think we can close this issue, correct? This was an investigation issue and we have already completed the implementation and merge.

[skabashnyuk]

issues is not closed because code related to it is not merged in master. We will do that when epic #1790 will be closed