[1.7.x] Wrapped migrations in a transaction only on DBs with transactional DDL.

    def __enter__(self):

        self.deferred_sql = []

        atomic(self.connection.alias, self.connection.features.can_rollback_ddl).__enter__()

        if self.connection.features.can_rollback_ddl:

            atomic(self.connection.alias).__enter__()

        return self

    def __exit__(self, exc_type, exc_value, traceback):

        if exc_type is None:

            for sql in self.deferred_sql:

                self.execute(sql)

        atomic(self.connection.alias, self.connection.features.can_rollback_ddl).__exit__(exc_type, exc_value, traceback)

        if self.connection.features.can_rollback_ddl:

            atomic(self.connection.alias).__exit__(exc_type, exc_value, traceback)

    # Core utility functions