More reliable stale items

[danialgood]

It seems currently stale values are resurrected via shd get_stale which is not to be relied on.
This could be costly in cases which cache is expired and an answer is needed nevertheless.

Maybe a backup lrucache could be implemented alongside current structure that has a copy of recently accessed keys.

[thibaultcha]

Hi,

Our usage of get_stale should be appropriate given we handle its edge-cases.

This could be costly in cases which cache is expired and an answer is needed nevertheless.

I think this relates to #94, aka serving stale items while re-validating.

I'd be inclined to close this issue as a duplicate of #94, unless you see a different issue with get_stale itself.