Make GetDbIdentity pure virtual and also implement it for StackableDB, DBWithTTL

Summary: As title

Test Plan: make clean and make

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14469

include/rocksdb/db.h

@@ -295,10 +295,7 @@ class DB {
   // Sets the globally unique ID created at database creation time by invoking
   // Env::GenerateUniqueId(), in identity. Returns Status::OK if identity could
   // be set properly
-  virtual Status GetDbIdentity(std::string& identity) {
-    identity.clear();
-    return Status::OK();
-  }
+  virtual Status GetDbIdentity(std::string& identity) = 0;
 
  private:
   // No copying allowed

include/utilities/stackable_db.h

@@ -140,6 +140,10 @@ class StackableDB : public DB {
     return db_->DeleteFile(name);
   }
 
+  virtual Status GetDbIdentity(std::string& identity) {
+    return db_->GetDbIdentity(identity);
+  }
+
   virtual Status GetUpdatesSince(SequenceNumber seq_number,
                                  unique_ptr<TransactionLogIterator>* iter)
     override {