The POST request to /_migration/system_features returns after the migration task has been submitted, but before it actually begins. This means that an immediately subsequent call to GET /_migration/system_features may return MIGRATION_NEEDED rather than IN_PROGRESS. This, in fact, happens consistently in the Upgrade Assistant. We should find a way to make sure that a call to GET /_migration/system_features returns state relevant to the most recent POST request.

cc @sabarasaba