Asynchronous boundaries

[jroper]

We may want to provide the ability to introduce asynchronous boundaries into streams, such that different parts of a stream can have different execution contexts, buffering strategies, etc. One possibility to provide this is to support the following method:

public ProcessorBuilder<T, R> async(ReactiveStreamsEngine engine);

This would cause all the stages prior to this stage to be executed using the passed in engine (which may have a custom configuration), and means that each engine is free to define what customization of execution attributes it supports. All subsequent stages would be run by whatever engine is passed to the eventual build method (or, the next async method). The underlying mechanism for doing the async boundary would be to use the passed in engine to build the Processor or Publisher, and then connecting the newly built Processor or Publisher via the subscribe method when it's run.

[Emily-Jiang]

Based on @cescoffier 's comment, this is only related to upstream to downstream events.

Can add emitOn(Executor executor) on PublishBuilder and ProcessorBuilder