R1:4adaf53bf0cb

Dramatically increase cache hit rate for feed

Summary:
Ref T8631. The query plan for feed stories is really bad right now, because we miss caches we should be hitting:

- The workspace cache is stored at each query, so adjacent queries can't benefit from the cache (only subqueries). Feed has primarily sibling queries.
- There is no technical reason to do this. Store the workspace cache on the root query, so sibling queries can hit it.
- In `ObjectQuery`, we check the workspace once, then load all…