Refactor to Reactive Streams APIs

[donmendelson]

Refactor to Reactive Streams.

Reactive Streams is an initiative to provide a standard for asynchronous stream processing with non-blocking back pressure. This encompasses efforts aimed at runtime environments (JVM and JavaScript) as well as network protocols.

The Reactive Streams interfaces include Publisher, Subscriber, Subscription and Processor. The interfaces are conducive to asynchronous protocols, including Websocket and HTTP/2.

Multiple implementations already exist, and the interfaces have been proposed for Java 9 through JEP 266: More Concurrency Updates.