Support RxJava 2.x

RxJava has moved to a new major API in 2.x. The changes aren't large, but since we promise compat within a version and expose RxJava as part of our public interface (and have since 2014 or so), it will require either a significant new surface area (bucket.async2() ?) or a new major version of both the SDK and io-core.

Current thought is that it may be possible to provide 'opt-in' with something like the above maneuver for SDK 2.7/core 1.7 with a move to a new reactive core including at least RxJava 2 at a later date.