Edited Feature: Public query cache [246]

We should have a design for the public surface of the query cache, so that people can easily do things like:

- Checking whether a specific query has a matching entry in the query cache
- Checking how many entries are there in the cache
- Manually start a cache cleanup
- Create custom policies for the cache
- Plug-in their own cache

This API should also be usable for third party profiling tools.