Add an option to specify an index column to be used for delete trigger

In some cases, a unique column has an index, but not defined as UNIQUE intentionally, like for partition tables to avoid having the column in the partitioning key. Currently, pt-osc would not work if the new schema doesn't have any PK nor UK. It would be helpful to have a new option, like --delete-trigger-index, which would allow a user to specify an index which would be used in the delete trigger.