DB migration 209: Clean up child rows as well

Fixes bug 1252891

In order to add missing foreign key constraints, rows that reference a
nonexistant key need to be purged. Unfortunately, DB migration 209 does
not clean up any rows in child tables that themselves have a foreign key
constraint on the tables with the missing foreign keys. This can result
in an integrity violation causing the migration to fail.

When purging the rows, make sure to purge rows in any child tables first.

Change-Id: I25bd899383c72cb0670819b030759630ac89acaa
Merge "DB migration 209: Clean up child rows as well"