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_param.cc

@@ -1,13 +1,13 @@
 // Copyright by Contributors
 #include "../../../src/tree/param.h"
-
 #include "../helpers.h"
+#include <gtest/gtest.h>
 
 TEST(Param, VectorIOStream) {
   std::vector<int> vals = {3, 2, 1};
   std::stringstream ss;
   std::vector<int> vals_in;
-  
+
   ss << vals;
   EXPECT_EQ(ss.str(), "(3,2,1)");