Modulestore migrator performance

[kdmccormick]

Per @ormsbee :

Course import into a library is painfully slow. The current pattern is to add and publish components one by one, meaning that even if events are processed on a PublishLog basis, we’re doing hundreds to thousands of them per import. Can we draft all the changes first and then publish all at once?

The PR at openedx/openedx-platform#38508 should start to fix this, but we still have more to go. Investigate the performance and see how we can improve it further.