Updated visualization to use scheduler

Background
-----
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
-----
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!