Updated visualization to use scheduler

Background
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Follow on from [a previous pull request](https://github.com/DynamoDS/Dynamo/pull/2624) this pull request makes use of the two new ```AsyncTask``` classes: ```UpdateRenderPackageAsyncTask``` and ```AggregateRenderPackageAsyncTask```.

Notes to Reviewer
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Hi Ian, since you have just reviewed my previous pull request, it makes complete sense to have you look at these minor changes. I have added various comments in the source code, hopefully that helps you while you review them. Thanks in advance!
I know this is weird and we need a more consistent way to raise ```ResultsReadyToVisualize``` event (they should all be raised from a UI thread), so I have created [a separate task](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4906) for unifying this.