Change batch fields to ManyToMany batches field (breaks everything).
Remove outdates FIXMEs after discussion with @jerith (loading models one at a time actually isn't so bad because it avoids map-reduce calls).
Make tags list optional in batch_start.
Record batch ids in the ManyToMany batches list and look up batch using the batches index.
Fix batches key look-up in event storing.
Add tests for adding messages to multiple batches.
Hook up incomplete InboundMessage and OutboundMessage model migrators and add copies of old message models to tests.
Progress on migrators.
Fix migrators and add test for migrating messages without batches.
Add all_keys() to models.
Add start of migrator script.
Fix poorly written docstring.
Where locating msg_fields in message_store_migrators, replace needless iteration over items with iteration over keys.
Refactor migration methods to use some common utilities functions.
Progress reporter and better error handling for model migrator.
First model migrator tests.
Fix signature of base load method on manager base class.
Add tests for error cases and better checking of succcess.
Fix tests so they don't rely on being sorted.
Add dry-run option to model_migrator.