Variable refresh

Currently a fixed duration in refreshAfterWrite is supported. An evaluation per entry, similar to expireAfter(Expiry), would provide a more flexible approach. In #261 this would evaluate on create, read, and update. In #498, it was also requested to evaluate on the exceptional case, where the entry remains unchanged.

Implementation notes

A quirk in Guava (and replicated in Caffeine for compatibility) is that expireAfterWrite and expireAfterAccess may both be set and honored. As refreshAfterWrite piggybacks on the write timestamp, we would have to extend the codegen to 3 timestamps. That isn't desirable or useful, so instead we should restrict this specific combination and reuse the 2 timestamp variations already generated (e.g. for expireAfter and refreshAfterWrite). Setting both fixed and variable refresh should be disallowed. This approach only requires retrofitting methods onto existing generated cache entries to map to the correct timestamp field.

This issue consolidates #261, #272, #360, and #498. @denghongcai @eugenemiretsky, @anuraaga, @musiKk, @minwoox, @ikhoon, @barkanido, @wcpan