He-Pin(kerr)

refs: https://github.com/akka/akka/issues/22849 too

https://mvnrepository.com/artifact/org.scalatestplus/testng-7-5 only available for scalatest 3.2.x

@otto-dev Yes, but at first I need to get more feedback from the Akka team wheather this PR is acceptible, I like the idea of making `onComplete` optional and passing...

I will update with test from the javadsl side later.

@filosganga @weihsiu I have updated it with the resource like clean up, would you like to give some input,thanks.

refs: https://github.com/Kotlin/kotlinx.coroutines/issues/2642

I am writing some examples now and will update them later.

In Flux, it's named as [Flux#using](https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html#using-java.util.concurrent.Callable-java.util.function.Function-java.util.function.Consumer-)

I think is one is nearly finished?

> LGTM. Excellent work @hepin1989 ! Thank you for very detailed review