Switch code that uses Reactive Streams to favour the Java 9 Flow API

* We have PublisherSource and SubscriberSource but these are based on the org.reactivestreams API. We have other methods and classes to support the Java 9 equivalents but these involve delegating to the org.reactivestreams based classes.
* It seems like for performance reasons we should using code that uses the Java 9 classes directly instead of doing conversions.
* It's possible that the code is this way as it was easier when we had to support Java 8 to minimise the code that uses the Java 9 APIs

Start by locating PublisherSource and SubscriberSource, then trace the Java 9 equivalents and the delegating paths built on org.reactivestreams. Determine which code paths can use the Java 9 Flow API directly and how Java 8 support constrains the change. Done means the relevant paths no longer need conversions while existing compatibility and behavior remain intact.

Written by the indexing model from the issue text.