LP #1225577: pt-online-schema-change can silently drop rows

**Reported in Launchpad by Rob Smith last update 05-10-2017 02:02:41

When adding a unique index on a table with pt-online-schema-change, it silently drops rows that fail the unique constraint. There should be the option to check that the row counts are the same before swapping tables.

Yes, we should check to verify that the columns are unique before running, but there are times the tools insert non-unique rows while PTOSC is running, thus having a final check would be best.