Assert last reference before destroy ColumnFamilyData

Summary:
In ColumnFamilySet destructor, assert it hold the last reference to cfd before destroy them.

Closes #3112
Closes https://github.com/facebook/rocksdb/pull/3397

Differential Revision: D6777967

Pulled By: yiwu-arbug

fbshipit-source-id: 60b19070e0c194b3b6146699140c1d68777866cb

db/column_family.cc

@@ -1060,10 +1060,12 @@ ColumnFamilySet::~ColumnFamilySet() {
   while (column_family_data_.size() > 0) {
     // cfd destructor will delete itself from column_family_data_
     auto cfd = column_family_data_.begin()->second;
-    cfd->Unref();
+    bool last_ref __attribute__((__unused__)) = cfd->Unref();
+    assert(last_ref);
     delete cfd;
   }
-  dummy_cfd_->Unref();
+  bool dummy_last_ref __attribute__((__unused__)) = dummy_cfd_->Unref();
+  assert(dummy_last_ref);
   delete dummy_cfd_;
 }