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.

src/predictor/gpu_predictor.cu

@@ -439,7 +439,7 @@ struct ShapSplitCondition {
     if (isnan(x)) {
       return is_missing_branch;
     }
-    if (categories.Size() != 0) {
+    if (categories.Capacity() != 0) {
       auto cat = static_cast<uint32_t>(x);
       return categories.Check(cat);
     } else {
@@ -454,7 +454,7 @@ struct ShapSplitCondition {
     if (l.Data() == r.Data()) {
       return l;
     }
-    if (l.Size() > r.Size()) {
+    if (l.Capacity() > r.Capacity()) {
       thrust::swap(l, r);
     }
     for (size_t i = 0; i < r.Bits().size(); ++i) {
@@ -466,7 +466,7 @@ struct ShapSplitCondition {
   // Combine two split conditions on the same feature
   XGBOOST_DEVICE void Merge(ShapSplitCondition other) {
     // Combine duplicate features
-    if (categories.Size() != 0 || other.categories.Size() != 0) {
+    if (categories.Capacity() != 0 || other.categories.Capacity() != 0) {
       categories = Intersect(categories, other.categories);
     } else {
       feature_lower_bound = max(feature_lower_bound, other.feature_lower_bound);