Zip with backpressure support

If I understand the backpressure correctly, zip with backpressure is relatively simple: if the client requests N items, it is forwarded to all sources because they would need to produce N events each in order to get N events zipped. Since N may vary, bounded buffering may not be feasible. 

In addition, I'm not sure what type of typical backpressure tests should be written so I only included a simple one.

I've also fixed the unsynchronized read of ```requested``` in ```Subscriber.setProducer```.