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
That's what fixtures are for. It would be great if we didn't need to create yet another provider specific artifact when creating a new test collection. Simpler to just standardize on fixtures as the single place where configuration is performed. 

Some other potential disadvantages: 
- Derived types mean we are more likely to hit context caching issues like we did in the old stack.
- A single context type better tests our ability to work against multiple providers (from one context).