Refactor DMatrix to return batches of different page types (#4686)

* Use explicit template parameter for specifying page type.

tests/ci_build/print_r_stacktrace.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# To be called when R package tests have failed
+
+set -e
+
+if [ -f "xgboost/xgboost.Rcheck/00install.out" ]; then
+  echo "===== xgboost/xgboost.Rcheck/00install.out ===="
+  cat xgboost/xgboost.Rcheck/00install.out
+fi
+
+if [ -f "xgboost/xgboost.Rcheck/00check.log" ]; then
+  printf "\n\n===== xgboost/xgboost.Rcheck/00check.log ====\n"
+  cat xgboost/xgboost.Rcheck/00check.log
+fi
+
+
+# Produce error code to interrupt Jenkins pipeline
+exit 1