stale-while-revalidate allows resources that are accessed often to be refreshed asynchronously.

This is implemented by a new freshness state that returns the cached response, but triggers an asynchronous request that gets stored in the cache.

## Example

If we need to call an external service to generate short-lived user JWT tokens (e.g., valid for a minute)

Imagine this request is slow and needs to be cached so the app is more responsive. However I we don't want the app to hang every minute when the JWT token expires, so it needs to be refreshed asynchronously.

Instead of returning `cache-control: max-age=60`, it can now return `cache-control: max-age=30, stale-while-revalidate=30`, so that JWT tokens for users making frequent requests are regenerated every ~30s.

(If the user stays away for over a minute the next request will be slower)

…ke PRs

Add go.mod, rename package

Stale while revalidate