[ofiwg] send/recv "credits"

[Hefty, Sean]

> So far support this has been mostly Jason and me talking with app-writer
> hats on claiming that this is an app-driven request, and Sean telling us
> to learn to write better apps :).

First, I use a credit based mechanism in rsockets, which would be terribly convenient to keep.  Second, I did propose a solution, it was just based on returning attributes to the app, so that the app can track its usage.

Where I hesitate is the introduction of an API which could potentially introduce a performance hit, or require that the provider do additional calculations (even if it's just ++) when it may not be necessary.

Additionally, I don't want to assume or force a completion ordering model on the provider.  Reliable unconnected endpoints may not adhere to this model.

- Sean