Source.unfoldAsync
Just like unfold but the fold function returns a Future .
Description
Just like unfold but the fold function returns a Future which will cause the source to complete or emit when it completes.
Can be used to implement many stateful sources without having to touch the more low level GraphStage API.
emits when there is demand and unfold state returned future completes with some value
completes when the future returned by the unfold function completes with an empty value