* Remove optional attributes parameter in favor of withAttributes
* Overload explosion when trying to add attributes to ZipWith.
* Aligned with Flow and Source.
* and rename the factory for ActorPublisherSource,
from Source.apply to Source.actorPublisher
* including internal buffer, with OverflowStrategy
* support to complete/fail stream
`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
* Remove name parameter (no overloads), naming is performed using `.withAttributes` or the new
convenience `.named`. Those adds the OperationAttribute.Name and also change the name of the
shape Inlet and Outlet.
* Remove Source/Sink parameter list for 0 parameter methods,
this allows usage of `Sink.head` instead of `Sink.head()`
* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
