LP #1203946: Add Post Alter swap/drop Only Functionality for pt-online-schema-change

**Reported in Launchpad by Jervin R last update 02-08-2013 19:19:10

In certain cases like pt-online-schema-change failing the drop/swap phases after all rows has been copied probably due to MDL locking, we should be able to do this phase manually via the tool itself as well. If the triggers are left intact and the old and new tables are intact and consistent, we can make use of such functionality to not put to waste the time it took for the row copying to complete specially if the table is very large to do over again.