Dispatch context

Some neat features will become available if we can have knowledge about whether we're operating in the context of a Dispatch operation. For example:
* Propagating the CorrelationId for an operation (implemented)
* A "Bus.Reply" concept for things like reporting progress of long-running orchestrations
* Others I haven't thought of right now... :wink:

Please consider. :smile:
All good. I have a local tweak and I'll push it shortly.