automatic recommendation caching part 1

This implements the basic feature of providing an automatic recsys-side reclist caching mechanism, which benefits all recsystems using this framework. This should help with performance issues on all recsystems without requiring any backend-side support for caching.

I would still like to make it easier for competitors to provide their own custom settings, both for the caching and for other settings (e.g. max articles to cache) but that is lower priority for right now and will come in a separate change.