Seamless scaleout for both callbacks and pubsub

This will fix #35 

### Plan of attack

* [x] Failing acceptance test for pubsub
* [x] Failing acceptance test for callbacks
* [x] Add support for multi dequeue
* [x] Add header and logic to allow senders to control the reply to queue to use
* [x] Add opt out override setting for the multi dequeue

cc @johnsimons 
This needs to be modified in the core, right?