Varnish 3 configuration

Shared caches are pretty strict when it comes to caching or not a request. This is good, otherwise it would cache requests that are not meant to be cached.

However, there is some things that can be done to improve cacheability:

1. Ignore Google Analytics cookies
2. Remove empty Cookie line
3. Normalize Accept-Encoding header
4. Allow replying with a stale response if the backend is slow

Bonus:

1. Remove some headers to reduce header size and hide some details about the server (security).
2. Add a debug header to help understand why a request is cached or not.