Serialized backpressure support proposal.

For #1671

It took me a while to get it working and I'm not sure what tests to write to check backpressure behavior besides the request 5s from a sequence of 15.

In addition, it was a bit confusing when to call onStart, what exactly the setProducer call supposed to do and what to do when noone calls setProducer from upstream. 
I don't grok this method yet as it is rather complicated so I'm going to have to come back to this another time ...