Evaluate better approach for resolving owner/lastModifier metadata when user does not exist

Note: waiting for confirmation of issue before moving forward. Will update ticket then.

Context

In a recent support ticket, a customer was experiencing a very long loading time for /entities. This customer has a lot of entities and muplitple engines connected to Optimize. After investigation we determined that the cause for this delay was the resolving of owner and lastModifier ID to userName done in EntityRestMapper.prepareRestResponse in connection with a recently deleted user. Because the user did not exist in the cache, Optimize queried each engine for the existence of this user every time the entities page was loaded, for every entity this user is owner/lastModifier of.

We should evaluate a solution to this issue. Potential options are:

Add more logging so that its more obvious when Optimize repeatedly cannot find certain users in the engine (this doesnt solve the issue but it helps debugging. If the removed user is owner of an entity, manual updates are still required)
Add a config flag to turn this resolving of user metadata on/off (not a great fix but an option if users don't care about seeing this info in the UI and happen to have a frequently changing userbase)
Add logic to update owner/lastModifier when Optimize cannot find this user in the engine (though this means Optimize would touch sensitive entity data, and may result in entity visibility changes when owner field is affected)

Or some better compromise.

ATs

a solution for above issue is found and implemented
the setup described in the support ticket does not result in slow performance anymore

Testing Notes

This issue is hard to reproduce. Testing steps will depend on the implemneted solution --- update once decision made.

Links

### Pull Requests

^{:robot: This issue is automatically synced from: source}

camunda / issues