LightGBM

[python] [dask] add initial dask integration (#3515)

* migrated implementation from dask/dask-lightgbm

* relaxed tests

* tests skipped in case that MPI is used

* fixed python 2.7 import + tests disabled on windows

* python < 3.6 is not supported in tests

* tests enabled only for linux

* tests disabled for mpi interface

* dask version pinned to >= 2.0

* added @jameslamb as code owner

* added missing pandas dependency

* code refactoring, removed code duplication - lightgbm.dask.LGBMClassifier.fit is the same as lightgbm.dask.LGBMRegressor.fit

* fixed refactoring

* code deduplication - fit method moved into mixin class

* fixed CODEOWNERS

* removed unnecessary import

* skip the module execution on python < 3.6 and on platform different than linux.

* removed skip for python < 3.6

* review comments

* removed noqa, renamed API classes, renamed local variables