Programmatically support delivery on Actor

[cjslep]

Right now an S2S application cannot easily programmatically trigger pub to do a delivery without faking a request to the PostOutbox method.

Introduce a FederatingActor that embeds Actor and will expose a new Deliver method. No more fake requests needed.

[cjslep]

d535bc9 solves this while maintaining backwards compatibility and without allowing an Actor to be cast to FederatingActor when created with C2S-only support.