Cleanup to prepare for using mmap pointer in external memory. (#9317)

- Update SparseDMatrix comment.
- Use a pointer in the bitfield. We will replace the `std::vector<bool>` in `ColumnMatrix` with bitfield.
- Clean up the page source. The timer is removed as it's inaccurate once we swap the mmap pointer into the page.

tests/cpp/common/test_column_matrix.cc

@@ -83,7 +83,9 @@ template <typename BinIdxType>
 void CheckColumWithMissingValue(const DenseColumnIter<BinIdxType, true>& col,
                                 const GHistIndexMatrix& gmat) {
   for (auto i = 0ull; i < col.Size(); i++) {
-    if (col.IsMissing(i)) continue;
+    if (col.IsMissing(i)) {
+      continue;
+    }
     EXPECT_EQ(gmat.index[gmat.row_ptr[i]], col.GetGlobalBinIdx(i));
   }
 }