[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.
by default, any sane proxy should not cache when cookies exist. but indeed this is misleading, you can change the configuration to make them cache. when you know what you are doing, that can even make sense ;-)