Add LiskRawMigrationIterator

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