Make the typing for `IncrementalCache` more explicit. With streaming, we’ll want to stream page data as well as HTML. This is a bit complicated now because we’re overloading `pageData` for both redirects and pages.
This PR makes the different types explicit. With streaming, the data for redirects is synchronously available, while the data for pages will become a stream.
A follow up PR will add a “stream through” cache in front of `IncrementalCache`
