Refactor Props's async logic to use Reactive Streams

[MihaiBojin]

Describe the solution you'd like

• the current async logic works but is custom
• there is value in making the interface compatible with org.reactivestreams and especially to support backpressure

Additional context

• register Subscriber<T> instead of Consumer<T>, Consumer<Throwable>
• each Prop<T> implements Publisher<T>
• alternatively examine java.util.concurrent.Flow (and related classes) as a possible interface