Cache invalidation mechanism

[xhacker]

We should keep cache for like...3 month?

[aquarhead]

I think we can try to do an exponential expiry, and have a threshold (like 3month) after which we cache it forever? (or fixed to account for upstream update) The progress will reset if upstream response doesn't match cached result.

(Maybe) need to add a small randomization to avoid refreshing all the things at the same.