Refactoring Northwind and OptimisticConcurrency functional tests

Split Northwind model into separate files and added a context class
Avoiding sharing any state between the contexts that mutate the model or the data
Renamed provider-specific test classes to include the provider name
Cleaning leftover databases
Nate added this. I don't think this would cause any failures. I think that the perf is what we should be worrying about. At least in SQL GUIDs destroy index perf, but ATS might not be as susceptible to this. Filed #961