This is a small polishing on the storage migration and storage rollback
rake tasks. By aborting a migration while a rollback is already
scheduled we want to prevent unexpected consequences.
Specs were reviewed and improved to better cover the current behavior.
There was some standardization done as well to facilitate the
implementation of the rollback functionality.
StorageMigratorWorker was extracted to HashedStorage namespace were
RollbackerWorker will live one as well.