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.
Rollback is done similar to Migration for the Hashed Storage.
It also shares the same ExclusiveLease key to prevent both happening
at the same time.
All Hashed Storage related workers now share the same queue namespace
which allows for assigning dedicated workers easily.
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.
