Merge 0.20.5 into 1.x

Merge the following:

- https://github.com/ReactiveX/RxJava/pull/1686 Fix Rx serialization bug in takeUntil again and the concurrent issue in BufferUntilSubscriber
- https://github.com/ReactiveX/RxJava/pull/1701 Fix the compose generics
- https://github.com/ReactiveX/RxJava/pull/1712 Fixing regression in mergeDelayError