Dmatrix refactor stage 2 (#3395)

* DMatrix refactor 2

* Remove buffered rowset usage where possible

* Transition to c++11 style iterators for row access

* Transition column iterators to C++ 11

src/tree/updater_gpu_hist.cu

@@ -19,6 +19,7 @@
 #include "../common/hist_util.h"
 #include "../common/host_device_vector.h"
 #include "../common/timer.h"
+#include "../common/common.h"
 #include "param.h"
 #include "updater_gpu_common.cuh"
 
@@ -803,10 +804,8 @@ class GPUHistMaker : public TreeUpdater {
 
     reducer_.Init(device_list_);
 
-    dmlc::DataIter<SparsePage>* iter = dmat->RowIterator();
-    iter->BeforeFirst();
-    CHECK(iter->Next()) << "Empty batches are not supported";
-    const SparsePage& batch = iter->Value();
+    auto batch_iter = dmat->GetRowBatches().begin();
+    const SparsePage& batch = *batch_iter;
     // Create device shards
     shards_.resize(n_devices);
     dh::ExecuteIndexShards(&shards_, [&](int i, std::unique_ptr<DeviceShard>& shard) {
@@ -828,8 +827,8 @@ class GPUHistMaker : public TreeUpdater {
         shard->InitCompressedData(hmat_, batch);
       });
     monitor_.Stop("BinningCompression", dist_.Devices());
-
-    CHECK(!iter->Next()) << "External memory not supported";
+    ++batch_iter;
+    CHECK(batch_iter.AtEnd()) << "External memory not supported";
 
     p_last_fmat_ = dmat;
     initialised_ = true;