Adapt Akka Future to Java 8 CompletableFuture

[HaloFour]

This is a much simpler approach. I've updated the AkkaFutureAdapter class to instead convert the Akka Future[T] to a Java 8 CompletableFuture[T].

I've kept the Sirius interface the same as in that it continues to return Future[T] which would eliminate the potential for breaking a consumer which implements that interface directly, but that does put additional onus on the consumer to determine if the returned Future[T] is a CompletableFuture[T] or not.

[CLAassistant]

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