dedupe pending revalidation requests (#58990)
### What?
We currently dedupe fetch requests, but if those fetch requests contain a `revalidate` time, when that window is expired all of those fetches will be invoked without deduping.
### Why?
We track revalidations on the `staticGenerationStore` but we don't have a way to dedupe them, as it's currently just an array. When the (patched) fetch is invoked and catches a stale entry, it'll push each fetch onto the `pendingRevalidates` array which will later be invoked via `Promise.all`.
### How?
This updates the shape of `pendingRevalidates` to be a map, that way we can reliably dedupe if we see a key that is already pending revalidation.
Closes NEXT-1744
[slack x-ref](https://vercel.slack.com/archives/C03S8ED1DKM/p1700836529460289)
