* Remove optional attributes parameter in favor of withAttributes
* Overload explosion when trying to add attributes to ZipWith.
* Aligned with Flow and Source.
`abstract class FlattenStrategy` was used in both javadsl and scaladsl, but the concrete concat
for the javadsl was in javadsl.FlattenStrategy and the concrete concat for the scaladsl is in
stream.FlattenStrategy.
Now there are separate FlattenStategy in scaladsl and javadsl packages and conversion
as we have in other places.
* replace JavaConverters with explicit methods
* remove asJava/asScala for FlattenStrategy
