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 @ikeough, 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!
Tiny optimization: If there are no Render Packages, don't raise raise ResultsReadyToVisualize