Support lazy refreshing of long-lived cache entries

[adamhoward56]

Hey team!

I have a use case where my cache expires entries hours after being written. I would like to lazily refresh these values to prevent staleness, i.e. if I retrieve an entry older than a few minutes, Moka will yield the stale value but also kick off a background process to compute the latest value.

The desired behavior would be adjacent to Guava's LoadingCache refreshAfterWrite functionality:

refreshAfterWrite will make a key eligible for refresh after the specified duration, but a refresh will only be actually initiated when the entry is queried.

Callers would need to provide (1) a function that Moka executes in the background to compute the latest cache value and (2) a parameter to define a staleness threshold after which a refresh should be performed.

It's easy to write a small wrapper that implements this behavior, but perhaps it would be more widely beneficial to include support for this directly in Moka. Curious if this could be considered/added to the roadmap, thanks!