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/logging.cc

@@ -4,9 +4,9 @@
  * \brief Implementation of loggers.
  * \author Tianqi Chen
  */
+#include <rabit/rabit.h>
 #include <xgboost/logging.h>
 #include <iostream>
-#include "./common/sync.h"
 
 #if !defined(XGBOOST_STRICT_R_MODE) || XGBOOST_STRICT_R_MODE == 0
 // Override logging mechanism for non-R interfaces