xgboost update

[sh1ng]

• Merged changes from upstream
• Testing on single and multiple gpu separately
• @pytest.mark.multi_gpu is suposed to beused for multi-gpu testing
• n_gpus was deprecated and as result a few changes to our wrapper
• Found dask, dask-cuda that are compatible

Related issues: https://github.com/dmlc/xgboost/issues/5003 https://github.com/dmlc/xgboost/issues/4988 https://github.com/rapidsai/dask-cuda/issues/166 https://github.com/dmlc/xgboost/issues/4987 https://github.com/dmlc/xgboost/pull/4990

[pseudotensor]

Will review soon. I'll basically diff dmlc head and this branch head and see how compares to diffs from bundle of commits from my last re-doing of the merge.

[sh1ng]

last integrated commit https://github.com/h2oai/xgboost/commit/185e3f19169d79b0910b848de5fe8055434e675e plus I had to make some changes on top of it manually(last merge was not resolved correctly all the places or merge algo not always correct when branches diverge a lot).