Sliding expiries #259

What problem does the feature solve?

Consider adding the option of a sliding expiry to cache items, as supported by both IMemoryCache and IDistributedCache in Microsoft.Extensions.*. This would allow cache items to be evicted before their TTL expired if they were not being actively used.

How would you use/interact with the feature? (if applicable)

The CacheSettings type could be extended to also define a SlidingExpiry property that could be used by a cache layer if supported by the underlying provider.