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/linear/updater_gpu_coordinate.cu

@@ -235,10 +235,8 @@ class GPUCoordinateUpdater : public LinearUpdater {
       row_begin = row_end;
     }
 
-    auto iter = p_fmat->ColIterator();
     CHECK(p_fmat->SingleColBlock());
-    iter->Next();
-    auto &batch = iter->Value();
+    const auto &batch = *p_fmat->GetColumnBatches().begin();
 
     shards.resize(n_devices);
     // Create device shards