Add ability to limit updates in migrations.
To make sure things are looking okay in the migration, we can run it
with a limit of something like 10 or 30. We can look at the output of
the migrated columns to see if they are correct. This should have no
effect on subsequently running the full migration.
Change-Id: I2c74879c8909c7938f994e1bd972d19325bc01f0
