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/metric/elementwise_metric.cc

@@ -4,11 +4,11 @@
  * \brief evaluation metrics for elementwise binary or regression.
  * \author Kailong Chen, Tianqi Chen
  */
+#include <rabit/rabit.h>
 #include <xgboost/metric.h>
 #include <dmlc/registry.h>
 #include <cmath>
 #include "../common/math.h"
-#include "../common/sync.h"
 
 namespace xgboost {
 namespace metric {

src/metric/multiclass_metric.cc

@@ -4,9 +4,9 @@
  * \brief evaluation metrics for multiclass classification.
  * \author Kailong Chen, Tianqi Chen
  */
+#include <rabit/rabit.h>
 #include <xgboost/metric.h>
 #include <cmath>
-#include "../common/sync.h"
 #include "../common/math.h"
 
 namespace xgboost {

src/metric/rank_metric.cc

@@ -4,10 +4,10 @@
  * \brief prediction rank based metrics.
  * \author Kailong Chen, Tianqi Chen
  */
+#include <rabit/rabit.h>
 #include <xgboost/metric.h>
 #include <dmlc/registry.h>
 #include <cmath>
-#include "../common/sync.h"
 #include "../common/math.h"
 
 namespace xgboost {