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/tree/updater_sync.cc

@@ -7,7 +7,6 @@
 #include <vector>
 #include <string>
 #include <limits>
-#include "../common/sync.h"
 #include "../common/io.h"
 
 namespace xgboost {