Range scan continue indicate whether the body is key only

Coming from the SDK it would be really useful to us if the response from a range scan continue contained an indicator as to whether the body contains only keys or keys+values. Maybe something like a byte in the extras of the packet.

The exact use-case I have in mind here is in gocbcore where create/continue/cancel are stateless. As things stand at the moment we require the user to tell us if they're only using keys in both create and continue, it'd be good if they only needed to do so in create and we could then detect the mode from the packet returned by continue.