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
"IMHO having provider-specific code to create the model outside the context is even further from what we advocate."

Being able to create the model outside the context is a fundamental "building block" API that is there to support scenarios just like this one. It allows for explicit, fine-grained control over the configuration of services for a specific context. Additionally, we still want the tests to run as fast and so needlessly re-initializing the entire stack per test seems like the wrong approach.