Support cache tagging

Tag your HTTP cache entries, and later invalidate cache entries by tags. This is sometimes called [Tagged Cache Invalidation](http://blog.kevburnsjr.com/tagged-cache-invalidation), a simpler version of [Linked Cache Invalidation](http://tools.ietf.org/html/draft-nottingham-linked-cache-inv-03). See also [this Drupal proposal](https://groups.drupal.org/node/297773).

For Varnish, this will come down to setting a custom header on the response and then banning on that header later. 

By the way, Varnish have [their own implementation called hashtwo](https://www.varnish-software.com/blog/advanced-cache-invalidation-strategies?utm_source=twitter&utm_medium=social&utm_content=393200) that's probably more efficient, but as it's a paid solution, it's less relevant to us.

See also  http://www.gossamer-threads.com/lists/varnish/misc/19732#19732