So this is working great so far.  This is a bit postgres specific, but this 
is how I did it:

1. Create "migration ids" table manually

    CREATE TABLE record_migration (
        id INT NOT NULL PRIMARY KEY,
        is_migrated BOOLEAN DEFAULT NULL
    );
    INSERT INTO record_migration (id) SELECT id FROM record_table;

2. In SqlAlchemy

Task runner can get a batch of IDS & lock them:
UPDATE record_migration 
SET is_migrated = False 
FROM 
( SELECT id from record_migration 
WHERE is_migrated IS NULL
ORDER BY id ASC 
LIMIT 100
) subq
WHERE record_migration.id = subq.id
RETURNING record_migration.id; 

        Then just loop the ids for migration using sqlalchemy 

And when done, mark the record as migrated.

UPDATE record_migration 
SET is_migrated = True
WHERE id = %s;

Each taskrunner has it's own batch and "locks" them with a `False` flag.  

After everything is done, I can go in and look for any records that are 
still `False` (most likely because of an uncaught exception in that batch) 
and set them to NULL for another migration run.

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