[WIP] Persistence

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/233
Implementing Model model ... FOR REVIEW