Update to using Rackspace.Threading library instead of duplicating functionality
Add additional library targeting .NET 4.5
Resolve additional documentation links

* Add link to Rackspace.Threading library in Additional Reference Documentation project
* Fix two incorrect references in documentation comment and conceptual topic file
Use DelayedTask.WhenAll to simplify waiting in tests
Simplify waiting operations using DelayedTask and TaskBlocks.While
Remove unnecessary ParallelExtensionsExtras code