Supporting classes for the new threading model

Background
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This pull request implements ```DynamoScheduler``` and ```AsyncTask``` classes that are required for the new threading model. The pull request does not represent a complete and working version of these classes, it is created in the interest of having multiple, smaller chunks of the larger redesign work.

The two new classes are not referenced anywhere in the existing code base.

Unit tests
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Few test cases are written to test ```TimeStampGenerator``` and ```AsyncTask``` classes. More test cases will be made possible with subsequent pull requests with ```DynamoScheduler``` and ```ISchedulerThread``` implemented more fully.

External references
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Documentation for the series of threading model redesign works [can be found here](https://docs.google.com/a/adsk-oss.com/document/d/17xmcMexPLmASaGJ89hKXzDC4EFmdOa5Mtl2gTG6hQ04/edit).


Hi @lukechurch and @ikeough, it seems like only two of you have seen the original document, perhaps these codes do not look too alien to you. And since @ke-yu and @Randy-Ma dropped by for a look, I might as well make use of their expertise and sharp eyes! Please have a look, thanks! :)

![Image of the Ironcat]
(https://octodex.github.com/images/ironcat.jpg)
Thanks guys, that's a good point. @lukechurch had suggested to use ```DateTime``` for debugging/visualization purposes, and then for production turn it into ```long``` for simplicity and performance. I guess like what Luke had done with the ```TimeStampOrdering```, I'll go ahead and make this a struct that internally contains either a ```DateTime``` or a ```long``` value depending on the need. I have updated the threading document to include [this action item](https://docs.google.com/a/adsk-oss.com/document/d/17xmcMexPLmASaGJ89hKXzDC4EFmdOa5Mtl2gTG6hQ04/edit#heading=h.bxyh9fj6c2p9) and address it in a subsequent PR (would love to keep this PR not more involved than it already is for now).