Provider specific migrations part 1:

1. Updated migration operations pre processors to be able to run at the very end of Diff instead of the begining of SQL generation. This will result in full sequences of migration operations specific to the provider.
2. Removed DatabaseModelModifier from production code, but still used in test code.
3. Updated tests. Removed some scenarios that were possible before because the migration operations could be altered  by the user before being passed to pre-processor, but are not possible anymore.

NOTE: The name PreProcessor should probably change. Because the input into the PreProcessor comes from the differ without being altered by the user, they can be simplified and even integrated into the provider specific diff logic, but I would prefer to do that a bit later, in order to avoid large changesets which are difficult to review.