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/io.h

@@ -9,9 +9,9 @@
 #define XGBOOST_COMMON_IO_H_
 
 #include <dmlc/io.h>
+#include <rabit/rabit.h>
 #include <string>
 #include <cstring>
-#include "./sync.h"
 
 namespace xgboost {
 namespace common {