Create interface for value exchange

Part of TCO entails measuring the impact of using channels for value exchange and finding possible alternatives.
Even though measurements so far do not indicate that channels are actually an issue, if we want to measure alternatives, we have to create an interface for value exchange, and implement that interface using our current channel mechanism.
This will allow us to test different implementations later on if we so wish.
There will be no change in behaviour to how things work currently - just formalising them in an explicit interface