Add generic dropper for duplicate rows

Add generic function that help us to drop duplicated rows from table,
before creating unique constarints. It is very usefull in migrations,
because it will allow DBA to make migrations without IntegrityError
exceptions.

It deletes all duplicates of rows except row with max value in id column.

blueprint db-unique-keys

Change-Id: I7e48d3eb78cf9a5d30752541b143b0c9fa1f838a
Merge "Add generic dropper for duplicate rows"