We need a way to run a command for all data in the cache (while under a lock) but also return a value for the data processed.

There are a few different APIs we could do for this having a Cache<K, V>

blocking variant

<R> Map<K, R> evalAll(BiFunction<Cache<K, V>, ? super CacheEntry<K, V>, R> biFunction)

back pressure aware variant - this would have to be a hot observer on subscribe

<R> Publisher<R> evalAll(BiFunction<Cache<K, V>, ? super CacheEntry<K, V> R> biFunction)