* Thrust 1.17 removes the experimental/pinned_allocator.
When xgboost is brought into a large project it can
be compiled against Thrust 1.17+ which don't offer
this experimental allocator.
To ensure that going forward xgboost works in all environments we provide a xgboost namespaced version of
the pinned_allocator that previously was in Thrust.
* Implement `MaxCategory` in quantile.
* Implement partition-based split for GPU evaluation. Currently, it's based on the existing evaluation function.
* Extract an evaluator from GPU Hist to store the needed states.
* Added some CUDA stream/event utilities.
* Update document with references.
* Fixed a bug in approx evaluator where the number of data points is less than the number of categories.
