MemTable::PostProcess() can skip updating num_deletes if the delta is 0

Summary: In many use cases there is no deletes. No need to pay the overhead of atomically updating num_deletes.

Test Plan: Run existing test.

Reviewers: ngbronson, yiwu, andrewkr, igor

Reviewed By: andrewkr

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60555

db/memtable.h

@@ -232,8 +232,10 @@ class MemTable {
     num_entries_.fetch_add(update_counters.num_entries,
                            std::memory_order_relaxed);
     data_size_.fetch_add(update_counters.data_size, std::memory_order_relaxed);
-    num_deletes_.fetch_add(update_counters.num_deletes,
+    if (update_counters.num_deletes != 0) {
-                           std::memory_order_relaxed);
+      num_deletes_.fetch_add(update_counters.num_deletes,
+                             std::memory_order_relaxed);
+    }
     UpdateFlushState();
   }