Synchronized SubscriptionList

As per discussion in https://github.com/Netflix/RxJava/issues/1383 ... despite the performance hit.
ScalarSynchronousObservable Perf
Remove Subscriber/Filter Dead Code

Removing code that would never get executed.
ring buffer
Progress on merge ... now supports most downstream backpressure.

- Parallel operator doesn't work with backpressure right now
- 1 use case (and unit test) is failing on merge