remove reactivestreams dependency?

* My understanding is that reactivestreams dependency is only really needed to support Java 8 users
* Java 9+ users can use https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Flow.html and related classes
* In theory, users who use Java 9+ might still want org.reactivestreams support too but is it best to just move on?

wdyt? @raboof @He-Pin @mdedetrich