From the names, it isn't clear why we wouldn't use `DatasetOutputCommitter` for a `RandomAccessDataset`. I think we should call this `MergeOutputCommitter` or `FileSystemOutputCommitter` (it is tied to the FS `Accessor`) and have a comment explaining what's going on for now. We should keep an eye on this because we may have different `Dataset` impls with specific committers, like accumulo or versioned datasets.