2.1.15 release preparations

[akarnokd]

Version 2.1.15 - June 22, 2018 (Maven)

API changes

• Pull 6026: Add blockingSubscribe overload with prefetch amount allowing bounded backpressure.
• Pull 6052: Change {PublishSubject|PublishProcessor}.subscribeActual to protected. They were accidentally made public and there is no reason to call them outside of RxJava internals.

Documentation changes

• Pull 6031: Inline CompositeDisposable JavaDoc.
• Pull 6042: Fix MulticastProcessor JavaDoc comment.
• Pull 6049: Make it explicit that throttleWithTimout is an alias of debounce.
• Pull 6053: Add Maybe marble diagrams 06/21/a
• Pull 6057: Use different wording on blockingForEach() JavaDocs.
• Pull 6054: Expand {X}Processor JavaDocs by syncing with {X}Subject docs.

Performance enhancements

• Pull 6021: Add full implementation for Single.flatMapPublisher so it doesn't batch requests.
• Pull 6024: Dedicated {Single|Maybe}.flatMap{Publisher|Observable} & andThen(Observable|Publisher) implementations.
• Pull 6028: Improve Observable.takeUntil.

Bugfixes

• Pull 6019: Fix Single.takeUntil, Maybe.takeUntil dispose behavior.
• Pull 5947: Fix groupBy eviction so that source is cancelled and reduce volatile reads.
• Pull 6036: Fix disposed LambdaObserver.onError to route to global error handler.
• Pull 6045: Fix check in BlockingSubscriber that would always be false due to wrong variable.

Other changes

• Pull 6022: Add TCK for MulticastProcessor & {0..1}.flatMapPublisher
• Pull 6029: Upgrade to Gradle 4.3.1, add TakeUntilPerf.
• Pull 6033: Update & fix grammar of DESIGN.md

[vanniktech]

Add full implementation for Single.flatMapPublisher so doesn't batch requests.

Seems like there's missing an it.

[akarnokd]

Closing via v2.1.15