Add Pop with subscription support?

[gavin-norman-sociomantic]

(Most of the work will be in dmqproto, just adding the issue here as a place-holder, for now.)

In the future, when most consumers have been moved over to using subscriber names, the Pop request (which currently cannot work on a subscribed channel) will become defunct. We do have possible use cases for Pop (e.g. when the user wants to strictly limit the input flow), so it'd be nice to get it working with subscribed channels.

It seems like simply adding an optional subscriber name to the Pop request (the same as Consume) should do the trick.