Blockchain: add get_db() accessor, needed for blockchain_import

This handling may be changed in the future.

src/cryptonote_core/blockchain.h

@@ -145,6 +145,11 @@ namespace cryptonote
     void set_enforce_dns_checkpoints(bool enforce);
     bool update_checkpoints(const std::string& file_path, bool check_dns);
 
+    BlockchainDB* get_db()
+    {
+      return m_db;
+    }
+
   private:
     typedef std::unordered_map<crypto::hash, size_t> blocks_by_id_index;
     typedef std::unordered_map<crypto::hash, transaction_chain_entry> transactions_container;