Summary: Ref T2222. I need this to migrate the Differential comment tables, because they are large. We have the similar `LiskMigrationIterator` already, but it won't work here because I intend to destroy the original objects after migrating them.
Test Plan:
Wrote a script to iterate over the `differential_comment` table, got reasonable output:
{P869}
Reviewers: btrahan
Reviewed By: btrahan
CC: chad, aran
Maniphest Tasks: T2222
Differential Revision: https://secure.phabricator.com/D6264
