Reduce iterator deletion overhead

Summary:
After introducing Iterator::PinData(), we have extra overhead of deleting the pinned iterators that we track in a std::set
This patch avoid inserting to the std::set if we have only one iterator (normal use case when no iterators are pinned)

Before this change
```
DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="newiterator" --db="/tmp/rocksdbtest-8616/dbbench" --use_existing_db --disable_auto_compactions
newiterator  :       1.006 micros/op 994013 ops/sec;
newiterator  :       0.994 micros/op 1006295 ops/sec;
newiterator  :       0.990 micros/op 1010422 ops/sec;
```

After change

```
DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="newiterator" --db="/tmp/rocksdbtest-8616/dbbench" --use_existing_db --disable_auto_compactions
newiterator  :       0.754 micros/op 1326588 ops/sec;
newiterator  :       0.759 micros/op 1317394 ops/sec;
newiterator  :       0.691 micros/op 1446704 ops/sec;
```

Test Plan: make check -j64

Reviewers: yhchiang, rven, anthony, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D52761

table/iterator_wrapper.h

@@ -94,20 +94,9 @@ class IteratorWrapper {
     return iters_pinned_ && iter_->IsKeyPinned();
   }
 
-  void DeletePinnedIterators(bool is_arena_mode) {
-    for (auto it : pinned_iters_) {
-      if (!is_arena_mode) {
-        delete it;
-      } else {
-        it->~InternalIterator();
-      }
-    }
-    pinned_iters_.clear();
-  }
-
   void DeleteIter(bool is_arena_mode) {
-    if (iter_) {
-      pinned_iters_.insert(iter_);
+    if (iter_ && pinned_iters_.find(iter_) == pinned_iters_.end()) {
+      DestroyIterator(iter_, is_arena_mode);
     }
     DeletePinnedIterators(is_arena_mode);
   }
@@ -132,6 +121,21 @@ class IteratorWrapper {
     }
   }
 
+  void DeletePinnedIterators(bool is_arena_mode) {
+    for (auto it : pinned_iters_) {
+      DestroyIterator(it, is_arena_mode);
+    }
+    pinned_iters_.clear();
+  }
+
+  inline void DestroyIterator(InternalIterator* it, bool is_arena_mode) {
+    if (!is_arena_mode) {
+      delete it;
+    } else {
+      it->~InternalIterator();
+    }
+  }
+
   InternalIterator* iter_;
   // If set to true, current and future iterators wont be deleted.
   bool iters_pinned_;