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 disagree. assume i have one url `/_fragment/user/toolbar` that renders the user name into every page. the rest of the content does not vary on cookie, but this one url does. and its embedded with ESI into every page. then we would want to cache this url as a variant for every single user to avoid reloading it all the time. once you log out, your session will be destroyed, meaning the session cookie changes and thus you don't see your username anymore.

obviously, you need to clean the cookie string for this to work, you don't want any js cookies irrelevant for the backend in here.