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.

tests/cpp/tree/test_refresh.cc

@@ -0,0 +1,57 @@
+/*!
+ * Copyright 2018 by Contributors
+ */
+#include "../helpers.h"
+#include "../../../src/common/host_device_vector.h"
+#include <xgboost/tree_updater.h>
+#include <gtest/gtest.h>
+#include <vector>
+#include <string>
+#include <memory>
+
+namespace xgboost {
+namespace tree {
+
+TEST(Updater, Refresh) {
+  int constexpr n_rows = 8, n_cols = 16;
+
+  HostDeviceVector<GradientPair> gpair =
+      { {0.23f, 0.24f}, {0.23f, 0.24f}, {0.23f, 0.24f}, {0.23f, 0.24f},
+        {0.27f, 0.29f}, {0.27f, 0.29f}, {0.27f, 0.29f}, {0.27f, 0.29f} };
+  auto dmat = CreateDMatrix(n_rows, n_cols, 0.4, 3);
+  std::vector<std::pair<std::string, std::string>> cfg {
+    {"reg_alpha", "0.0"},
+    {"num_feature", std::to_string(n_cols)},
+    {"reg_lambda", "1"}};
+
+  RegTree tree = RegTree();
+  tree.InitModel();
+  tree.param.InitAllowUnknown(cfg);
+  std::vector<RegTree*> trees {&tree};
+  std::unique_ptr<TreeUpdater> refresher(TreeUpdater::Create("refresh"));
+
+  tree.AddChilds(0);
+  int cleft = tree[0].LeftChild();
+  int cright = tree[0].RightChild();
+  tree[cleft].SetLeaf(0.2f, 0);
+  tree[cright].SetLeaf(0.8f, 0);
+  tree[0].SetSplit(2, 0.2f);
+
+  tree.Stat(cleft).base_weight = 1.2;
+  tree.Stat(cright).base_weight = 1.3;
+
+  refresher->Init(cfg);
+  refresher->Update(&gpair, dmat->get(), trees);
+
+  bst_float constexpr kEps = 1e-6;
+  ASSERT_NEAR(-0.183392, tree[cright].LeafValue(), kEps);
+  ASSERT_NEAR(-0.224489, tree.Stat(0).loss_chg, kEps);
+  ASSERT_NEAR(0, tree.Stat(cleft).loss_chg, kEps);
+  ASSERT_NEAR(0, tree.Stat(1).loss_chg, kEps);
+  ASSERT_NEAR(0, tree.Stat(2).loss_chg, kEps);
+
+  delete dmat;
+}
+
+}  // namespace tree
+}  // namespace xgboost