* Cleanup some pylint errors.
* Cleanup pylint errors in rabit modules.
* Make data iter an abstract class and cleanup private access.
* Cleanup no-self-use for booster.
- Add user configuration.
- Bring back to the logic of using scheduler address from dask. This was removed when we were trying to support GKE, now we bring it back and let xgboost try it if direct guess or host IP from user config failed.
* [dask] Use `distributed.MultiLock`
This enables training multiple models in parallel.
* Conditionally import `MultiLock`.
* Use async train directly in scikit learn interface.
* Use `worker_client` when available.
