Fix counter for memtable updates

Summary:
Right now in `PutCFImpl` we always increment NUMBER_KEYS_UPDATED counter for both in-place update or insertion. This PR fixes this by using the correct counter for either case.
Closes https://github.com/facebook/rocksdb/pull/2986

Differential Revision: D6016300

Pulled By: miasantreble

fbshipit-source-id: 0aed327522e659450d533d1c47d3a9f568fac65d

db/memtable.cc

@@ -788,6 +788,7 @@ void MemTable::Update(SequenceNumber seq,
           assert((unsigned)((p + value.size()) - entry) ==
                  (unsigned)(VarintLength(key_length) + key_length +
                             VarintLength(value.size()) + value.size()));
+          RecordTick(moptions_.statistics, NUMBER_KEYS_UPDATED);
           return;
         }
       }

db/write_batch.cc

@@ -1034,7 +1034,6 @@ class MemTableInserter : public WriteBatch::Handler {
     } else if (moptions->inplace_callback == nullptr) {
       assert(!concurrent_memtable_writes_);
       mem->Update(sequence_, key, value);
-      RecordTick(moptions->statistics, NUMBER_KEYS_UPDATED);
     } else {
       assert(!concurrent_memtable_writes_);
       if (mem->UpdateCallback(sequence_, key, value)) {