ObjectLoaderPartLoaderStep
constructor(loader: ObjectLoader, partLoader: ObjectLoaderPartLoader<T>, inputQueue: PartitionedQueue<PipelineEvent<ObjectKey, ObjectLoaderPartFormatter.FormattedPart>>, outputQueue: PartitionedQueue<PipelineEvent<ObjectKey, ObjectLoaderPartLoader.PartResult<T>>>, taskFactory: LoadPipelineStepTaskFactory, stepId: String)