Discuss the usage of an object store in favor of an RDBMS to persist entities

This issue is imported from pivotal - Originaly created at Feb 11, 2020 by bstrooband

In order to persist entities I think we would be better of using an object store then a relational database. Now that we have dropped Edugain support we only query for entities on EntityId so it will be possible.

Some benefits:

• schema less (no migrations)
• we could update entities easily one-by-ony instead of all-at-once.
• we could get rid of doctrine which will require an extra release before cleanup (fields in selects based on annotations)
• The removal of doctrine prevents extensive hydration.
• better horizontal scaling
• multi language support would not require additional fields

Maybe it's too early to do now because the procedural style of Corto which wil make refactoring harder. So maybe Corto needs to be refactored out before doing this. But I would like to open up this discussion on forehand.