Enabled scheduler, disabled two cancellation related tests

Background
-----
This pull request enables the scheduler to assess the impact it has on unit test cases.

The scheduler is enabled through a compiler flag ```ENABLE_DYNAMO_SCHEDULER``` defined in the following projects:

    src/DynamoCore/DynamoCore.csproj
    src/DynamoRevit/DynamoRevit.csproj
    src/Libraries/DynamoWatch3D/Watch3D.csproj
    src/Libraries/Revit/RevitServices/RevitServices.csproj


It also disables the follow two unit test cases that are known to fail due to scheduler changes:

    RecordedTestsDSEngine.TestCancelExecution
    RecordedTestsDSEngine.TestCancelExecutionWhileLoop


These test cases fail due to the fact that task cancellation (for long running evaluation) is only planned to be [implemented in subsequent sprints](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4994).