R1:43935d5916f2

Don't cache resources we can't generate properly

Summary:
Fixes T10843. In a multi-server setup, we can do this:

- Two servers, A and B.
- You push an update.
- A gets pushed first.
- After A has been pushed, but before B has been pushed, a user loads a page from A.
- It generates resource URIs like `/stuff/new/package.css`.
- Those requests hit B.
- B doesn't have the new resources yet.
- It responds with old resources.
- Your CDN caches things. You now have a poisoned CDN: old data is…