MigrationProcess: StorageDescriptor<[], {
    child_items: number;
    progress_child: Enum<{
        Complete: undefined;
        LastKey: Binary;
        ToStart: undefined;
    }>;
    progress_top: Enum<{
        Complete: undefined;
        LastKey: Binary;
        ToStart: undefined;
    }>;
    size: number;
    top_items: number;
}, false>

Migration progress.

This stores the snapshot of the last migrated keys. It can be set into motion and move forward by any of the means provided by this pallet.