Remove the use of memset() in CPU 'hist' algorithm

[hcho3]

The use of memset() to initialize an array of structures is forbidden by CRAN. For now, we use XGBOOST_STRICT_R_MODE macro to selectively disable memset():

xgboost/src/common/hist_util.cc

Lines 665 to 671 in ea6b117

	void InitilizeHistByZeroes(GHistRow hist, size_t begin, size_t end) {
	#if defined(XGBOOST_STRICT_R_MODE) && XGBOOST_STRICT_R_MODE == 1
	std::fill(hist.begin() + begin, hist.begin() + end, tree::GradStats());
	#else // defined(XGBOOST_STRICT_R_MODE) && XGBOOST_STRICT_R_MODE == 1
	memset(hist.data() + begin, '\0', (end-begin)*sizeof(tree::GradStats));
	#endif // defined(XGBOOST_STRICT_R_MODE) && XGBOOST_STRICT_R_MODE == 1
	}

We should simplify the code by using std::fill() for all use cases, not just when XGBoost is compiled to use with R.

TODO. Run benchmark and measure performance impact of replacing memset() with std::fill().