This PR implements cache-aware server push using cookies.

ToDo:

• provide a configuration directive to turn on the feature (default should be OFF)
• provide a way to define the mime-types of files that need to be tracked
• implement a way to create positive list of the fingerprint that can be used to evict the negative ones (i.e. flags corresponding to files being removed or altered) from the client-side cookie
  • note: this is only necessary for cookie-based approach; when using SW we can build an accurate fingerprint on the client-side by iterating through the SW cache

part of #421