Refactoring OptimisticConcurrencyTest

  * Sharing the service provider
  * Creating the TestStore in the constructor and disposing in Dispose()
  * Using the same model for the same fixture
  * Moving OnModelCreating to the fixture and removing provider-specific contexts

Also fixing some broken tests
Filed #977
The convention needs to run as soon as the ModelBuilder is created so we could have the ModelBuilderFactory to call it to avoid invoking virtual methods from the constructor.
I don't see a big problem with the ModelBuilder being coupled to the type of the context. This would give us ability to support other conventions, perhaps some that use annotations on the context types or set properties.
However if we don't want that we could make the ModelBuilderFactory to perform the discovery.