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!
The node render package updates are async tasks. I think the scheduler processes these tasks in the order they arrive, and the "async" is referring to the fact that the scheduler thread is not blocking main.

Is it guaranteed that these visual updates will all be processed before the aggregate task is processed?