Migrate to RxJava2

[FroMage]

Possibly keep supporting RxJava1 but not in the Server API, only in resources.

[FroMage]

OK done, though this means the plugins/server APIs changed and are not backwards-compatible.

In particular, most methods that returned a Single<Void> now return Completable, which is an annoying type, but it helps to remember the following mappings:

• Single.just(null) -> Completable.completed()
• Single<Void>.map -> Completable.doAfterComplete
• Single<Void>.flatMap -> Completable.concatWith (but whatch out it doesn't take a Callable!!!!)

@k33g watch out ;)