Expose Cache based Publisher

We currently have cache based iterator that work irrespective of cache mode. We should expose something like Java10 Flow.Publisher or reactive-streams Publisher as well. This would be helpful when retrieving all entries from the cache in an asynchronous fashion. Currently you would have to create an iterator and iterate upon it, causing a dedicated user thread as well as buffering and context switching.

This can be used by enhanced listener API we hope to introduce as well when retrieving initial state.