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_bitfield.cc

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2019 XGBoost contributors
+/**
+ * Copyright 2019-2023, XGBoost contributors
  */
 #include <gtest/gtest.h>
 #include "../../../src/common/bitfield.h"
@@ -14,7 +14,7 @@ TEST(BitField, Check) {
                 static_cast<typename common::Span<LBitField64::value_type>::index_type>(
                     storage.size())});
     size_t true_bit = 190;
-    for (size_t i = true_bit + 1; i < bits.Size(); ++i) {
+    for (size_t i = true_bit + 1; i < bits.Capacity(); ++i) {
       ASSERT_FALSE(bits.Check(i));
     }
     ASSERT_TRUE(bits.Check(true_bit));
@@ -34,7 +34,7 @@ TEST(BitField, Check) {
       ASSERT_FALSE(bits.Check(i));
     }
     ASSERT_TRUE(bits.Check(true_bit));
-    for (size_t i = true_bit + 1; i < bits.Size(); ++i) {
+    for (size_t i = true_bit + 1; i < bits.Capacity(); ++i) {
       ASSERT_FALSE(bits.Check(i));
     }
   }