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.
It won't be called for SqlServer or SQLite but if someone writes a new provider that uses sequences and doesn't override the method they can get the exception. Will add a message like "Override to generate SQL for renaming a sequence."