Redundant calls when creating cells

[tdudgeon]

When a notebook is opened multiple unnecessary calls are made when creating a cell.
Using scatterplot as an example (but presumably similar things happen with other cells) when a notebook with a cell is opened this sequence of calls seems to happen in ScatterPlotCanvasItemPanel.java:

1. refreshPlotData() called from constructor -> causes dataset to be retrieved from service (and hence db)
2. buildPlotJs() called from renderHead() -> results is plot being built and then resized
3. refreshPlotData() then buildPlotJs() called from onExecute() -> causes dataset to be retrieved again from service
4. buildPlotJs() called from renderHead() [again] -> results is plot being built, resized and built again

The net effect is that the data is retrieved from the database twice and the cell contents are created three times and resized twice.
With large datasets and lots of cells this will start to have significant impact.
It should be possible to do this with the data only being retrieved once, and the cell contents created once and not resized.