The new ALTER TABLE facility allows to act on tables found in the MySQL
database before the migration happens. In this patch the only provided
actions are RENAME TO and SET SCHEMA, which fixes#224.
In order to be able to provide the same option for MS SQL users, we will
have to make it work at the SCHEMA level (ALTER SCHEMA ... RENAME TO
...) and modify the internal schema-struct so that the schema slot of
our table instances are a schema instance rather than its name.
Lacking MS SQL test database and instance, the facility is not yet
provided for that source type.
