LightGBM

Refactoring monotone constraints (linked to #2305) (#2717)

* Move monotone constraints to the monotone_constraints files.

* Add checks for debug mode.

* Refactored FindBestSplitsFromHistograms.

* Add headers.

* fix

* Update data_parallel_tree_learner.cpp

* simplify ComputeBestSplitForFeature

* Fix min / max issue.

* Remove duplicated check.

Co-authored-by: Guolin Ke <guolin.ke@outlook.com>