Only call the config server when needed

The config server has been correctly set up so that it can get the value for the crosshair X,Y from the `display.configuration` file. However testing on the beamline has shown that instead of only poking the config server as-needed (on opening the screen, on changing the zoom level, on moves) it continuously pokes it. This is a bit overkill and it makes it very difficult to read the logs and debug other issues.