Refactor Subscriber to support backpressure when using lift and bug fixes

I found `tail` and `toXXX` didn't work after adding backpressure. To solve it, I have to change `Subscriber` to `abstract class` so that `Subscriber`s can be chained and call `request` properly.
@samuelgruetter could you review this PR? I don't understand why you created a new `rx.Subscriber` here. Do I miss anything?