Refactor fast-hist, add tests for some updaters. (#3836)

Add unittest for prune.

Add unittest for refresh.

Refactor fast_hist.

* Remove fast_hist_param.
* Rename to quantile_hist.

Add unittests for QuantileHist.

* Refactor QuantileHist into .h and .cc file.
* Remove sync.h.
* Remove MGPU_mock test.

Rename fast hist method to quantile hist.

src/common/hist_util.h

@@ -11,7 +11,6 @@
 #include <limits>
 #include <vector>
 #include "row_set.h"
-#include "../tree/fast_hist_param.h"
 #include "../tree/param.h"
 #include "./quantile.h"
 
@@ -19,8 +18,6 @@ namespace xgboost {
 
 namespace common {
 
-using tree::FastHistParam;
-
 /*! \brief sums of gradient statistics corresponding to a histogram bin */
 struct GHistEntry {
   /*! \brief sum of first-order gradient statistics */
@@ -145,7 +142,7 @@ class GHistIndexBlockMatrix {
  public:
   void Init(const GHistIndexMatrix& gmat,
             const ColumnMatrix& colmat,
-            const FastHistParam& param);
+            const tree::TrainParam& param);
 
   inline GHistIndexBlock operator[](size_t i) const {
     return {blocks_[i].row_ptr_begin, blocks_[i].index_begin};