Cache filter requests

[hudson-newey]

At the moment we are caching GET and HEAD requests

However, we can improve the performance quite a lot if we start caching filter requests (which are not cached by default because we abuse http semantics to send queries with a POST method and the filter parameters in the body).

• either using immutable objects with well defined hash protocol for all filters
  bonus wins for structural equality
• or by controlling serialization ourselves with an interceptor, and then using payload as cache key, or hashing payload. Bonus more control over our serialization protocol (toJSON has been problematic in the past)