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.
Yes. Using `lift` to create a new operator will need this one, such as `tail`, `toMultimap`, `to[Col]`