Rough OC hookup (of a DummyOutputCommitter that does barely anything, but on Hadoop 2) is now this PR. I need at least to figure out some testing before i can pull, but it is enough to start further work. @Lynn Rozen for now you might base your branch on feature/4310-lakefs-oc-minimal-hookup instead of feature/lakefs-output-committer; it will end up there, so rebasing should be easy.