[WIP] Varnish cookbook session cookie handling

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #3881

This builds on top of #4627 but i wanted to keep it separate as there are open questions in here.
setting vary on cookie doesnt make sense if you dont try to https://github.com/symfony/symfony-docs/pull/4628#discussion-diff-21731929R58?