No easy way to go 'local'

From a performance point of view, it would be very beneficial to be a
NAOqi 'local module' to access the video buffer.

It remains to be seen if we can write a ROS nodes that also happen to be a
NAOqi local module (that would allow much better performances for the camera
node).
Only publish frames when someone is listening on the topic

This saves a lot of CPU on Nao when images are not needed.
Remove useless mutex