Added support for sequences to migrations.

Added the following migration operations: MoveSequence, RenameSequence, AlterSequence.
Specialized DatabaseBuilder and ModelDiffer for SqlServer because the annotations for sequences are only available there (should we move to Relational?)
In SqlServerModelDiffer the sequences are matched based on the matching columns.
Added unit tests.
I don't remember why they were not made abstract and I can't think of a reason why they shouldn't. Probably they had an implementation initially. You are right, abstract is better, will change to that.