unfriend ColumnFamilyData from VersionSet

Summary: as title

Test Plan:
make release
will run full test on all stacked diffs before committing

Reviewers: sdong, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27591

db/column_family.cc

@@ -304,8 +304,9 @@ ColumnFamilyData::~ColumnFamilyData() {
 
   if (dummy_versions_ != nullptr) {
     // List must be empty
-    assert(dummy_versions_->next_ == dummy_versions_);
-    delete dummy_versions_;
+    assert(dummy_versions_->TEST_Next() == dummy_versions_);
+    bool deleted __attribute__((unused)) = dummy_versions_->Unref();
+    assert(deleted);
   }
 
   if (mem_ != nullptr) {

db/version_set.cc

@@ -2933,6 +2933,9 @@ ColumnFamilyData* VersionSet::CreateColumnFamily(
   assert(edit->is_column_family_add_);
 
   Version* dummy_versions = new Version(nullptr, this);
+  // Ref() dummy version once so that later we can call Unref() to delete it
+  // by avoiding calling "delete" explicitly (~Version is private)
+  dummy_versions->Ref();
   auto new_cfd = column_family_set_->CreateColumnFamily(
       edit->column_family_name_, edit->column_family_, dummy_versions,
       cf_options);

db/version_set.h

@@ -292,10 +292,14 @@ class Version {
   // seconds/minutes (because of concurrent compactions).
   static const size_t kNumberFilesToSort = 50;
 
+  // Return the next Version in the linked list. Used for debug only
+  Version* TEST_Next() const {
+    return next_;
+  }
+
  private:
   friend class VersionSet;
   friend class DBImpl;
-  friend class ColumnFamilyData;
   friend class ForwardIterator;
   friend class InternalStats;