Tango extension image devices resubscribe at every frame

[tiagocoutinho]

The tango extension taurus.core.tango.img devices at each frame received subscribe and unsubscribe to configuration events.
In the frame handler code there is a temporary access to the image attribute which triggers this call.
As frames can arrive at a frequent pace this slows down considerable and, more importantly, triggers the tango-controls/pytango#292.

The fix should be simple: just keep a reference to the attribute in the device. I can make a PR