[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.
I actually don't quite understand this section yet. Is this so that I can cache this Response on a user-by-user (Cookie-by-Cookie) basis? Or is something else happening.