Make level iterator customzied too.

Customized BlockBasedTableIterator

Summary:
Use a customzied BlockBasedTableIterator to replace current implementation leveraging two level iterator. Hope the customized logic will make code easier to understand. As a side effect, we reduce the allocation for the data block iterator object, and avoid the virtual function call to it, because we can directly reference BlockIter, a final class. It also enabled further optimization.

The upper bound check is also moved from index block to data block. This implementation fits this iterator better. After the change, forwared iterator is slightly optimized to ensure we trim those iterators.

Test Plan: Run all existing tests using ASAN.

Simplify two-level-iterator

fix clang

Fix issues because of rebase

Address comments.

Fix a memory issue

db/forward_iterator.cc

@@ -383,14 +383,13 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
 
       if (!l0_iters_[i]->status().ok()) {
         immutable_status_ = l0_iters_[i]->status();
-      } else if (l0_iters_[i]->Valid()) {
-        if (!IsOverUpperBound(l0_iters_[i]->key())) {
-          immutable_min_heap_.push(l0_iters_[i]);
-        } else {
-          has_iter_trimmed_for_upper_bound_ = true;
-          DeleteIterator(l0_iters_[i]);
-          l0_iters_[i] = nullptr;
-        }
+      } else if (l0_iters_[i]->Valid() &&
+                 !IsOverUpperBound(l0_iters_[i]->key())) {
+        immutable_min_heap_.push(l0_iters_[i]);
+      } else {
+        has_iter_trimmed_for_upper_bound_ = true;
+        DeleteIterator(l0_iters_[i]);
+        l0_iters_[i] = nullptr;
       }
     }
 
@@ -417,15 +416,14 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
 
         if (!level_iters_[level - 1]->status().ok()) {
           immutable_status_ = level_iters_[level - 1]->status();
-        } else if (level_iters_[level - 1]->Valid()) {
-          if (!IsOverUpperBound(level_iters_[level - 1]->key())) {
-            immutable_min_heap_.push(level_iters_[level - 1]);
-          } else {
-            // Nothing in this level is interesting. Remove.
-            has_iter_trimmed_for_upper_bound_ = true;
-            DeleteIterator(level_iters_[level - 1]);
-            level_iters_[level - 1] = nullptr;
-          }
+        } else if (level_iters_[level - 1]->Valid() &&
+                   !IsOverUpperBound(level_iters_[level - 1]->key())) {
+          immutable_min_heap_.push(level_iters_[level - 1]);
+        } else {
+          // Nothing in this level is interesting. Remove.
+          has_iter_trimmed_for_upper_bound_ = true;
+          DeleteIterator(level_iters_[level - 1]);
+          level_iters_[level - 1] = nullptr;
         }
       }
     }