[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.
this is the actual question to answer what @greg0ire asks in #3881

can we do this? is there somewhere in the doc something on this topic? we should probably have that not in here but link from here...