Optimistic caching

Just a random idea, but we could return cached versions of requested resources and then use streaming rendering to update it if necessary. We should have an indicator showing that the resource hasn't fully loaded yet.

If the user agent doesn't support streaming rendering, don't use the cache.

No files, tests, or entry points are named. Start by locating the resource-rendering and caching entry points, then check how user-agent streaming support is detected. Done means cached resources can render first with a loading indicator, stream updates when supported, and avoid the cache when it is not.

Written by the indexing model from the issue text.