[Feature request] - Stale while revalidate caching

[juancarrey]

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It would be nice to have the feature for those caches that support TTL, that returns the stale value during revalidation of the cached instance, being able to react to the newer instance once retrieved updating the cache in the background for later hits.

This would speed up misses, allowing a stale object to be returned during revalidating time.

This could be optionally defined on the policies or cache options

Thank you :)