Replaced Settings with Gitlab.config for uniformity

The previous code works fine when running specs, but the API
does not exist outside that environment.

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.

# Conflicts:
#   spec/tasks/gitlab/backup_rake_spec.rb

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.

We are keeping compatibility with existing scheduled jobs.

Pool repositories are persisted in the database, and when the DB is
restored, the data need to be restored on disk. This is done by
resetting the state machine and rescheduling the object pool creation.

This is not an exact replica of the state like at the time of the
creation of the backup. However, the data is consistent again.

Dumping isn't required as internally GitLab uses git bundles which
bundle all refs and include all objects in the bundle that they require,
reduplicating as more repositories get backed up. This does require more
data to be stored.

Fixes https://gitlab.com/gitlab-org/gitaly/issues/1355