Implemented new scheduler thread classes

Background
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This pull request mainly implements two new scheduler threads as described in [the documentation](https://docs.google.com/a/adsk-oss.com/document/d/17xmcMexPLmASaGJ89hKXzDC4EFmdOa5Mtl2gTG6hQ04/edit):

1. ```DynamoSchedulerThread``` for standalone Dynamo
2. ```RevitSchedulerThread``` for Revit hosted Dynamo

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