Do not send the new protocol message to non-XL client.

Do not send the new protocol message to non-XL client.

The new message 'W' to report waited-for XIDs must not be sent to a non-XL
client since it's not capable of handling that and might just cause unpleasant
problems. In fact, we should change 'W' to something else since standard libpq
understands that message and hangs forever expecting more data. With a new
protocol message, it would have failed, thus providing a more user friend
error. But postponing that for now since we should think through implications
of protocol change carefully before doing that.