Attempt at SubscribeOn "Time Gap" Issue [Preview]

This is an attempt at solving the `subscribeOn` "time gap" issue that I found to be affecting unit tests and use cases such as `parallel`. 

More information and discussion is at https://github.com/Netflix/RxJava/issues/844

Currently I have made this work by having a trigger on `Subscriber.onSubscribe()` that I'm not thrilled with but want to show this for discussion.