Pass pointer to model parameters. (#5101)

* Pass pointer to model parameters.

This PR de-duplicates most of the model parameters except the one in
`tree_model.h`.  One difficulty is `base_score` is a model property but can be
changed at runtime by objective function.  Hence when performing model IO, we
need to save the one provided by users, instead of the one transformed by
objective.  Here we created an immutable version of `LearnerModelParam` that
represents the value of model parameter after configuration.

src/tree/param.h

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2014 by Contributors
+ * Copyright 2014-2019 by Contributors
  * \file param.h
  * \brief training parameters, statistics used to support tree construction.
  * \author Tianqi Chen

src/tree/updater_gpu_common.cuh

@@ -39,7 +39,7 @@ struct GPUTrainingParam {
 using NodeIdT = int32_t;
 
 /** used to assign default id to a Node */
-static const int kUnusedNode = -1;
+static const bst_node_t kUnusedNode = -1;
 
 /**
  * @enum DefaultDirection node.cuh