Implement support to handle DDL Statements with Flashback

The first implementation of the flashback feature only reverts DML based statements up to a point where a DDL statement was executed.

For the support of DDL changes in the server are needed.

• For ADD INDEX/COLUMN, or CREATE TABLE query, just drop the index, column, table when running Flashback.
• For DROP INDEX/COLUMN, or DROP TABLE query, will copy or rename the old table to a reserved database. When Flashback is running, I can drop new table, and rename the saved old table to original database.
• For TRUNCATE table, I just rename old table to reserved database and create a new empty table.