Since now NibbleIterator can fallback to nibble (from one chunk) pt-osc
can nibble tables even if due to unacurate stats, it choses one nible as
the initial nibbling method.
In the case we are trying to migrate a table with no PK nor an unique
key, pt-archiver was failing to select the correct rows for deletion.
This fix implemented here is to add ALL columns in the WHERE clause of
the DELETE command. This way, we are deleting only the exact same row we
just migrated but using columns instead of an index.
