protocol: pass blockchain cumulative difficulty when syncing

Not used yet.

src/cryptonote_core/blockchain.cpp

@@ -2038,6 +2038,7 @@ bool Blockchain::find_blockchain_supplement(const std::list<crypto::hash>& qbloc
   {
     resp.m_block_ids.push_back(m_db->get_block_hash_from_height(i));
   }
+  resp.cumulative_difficulty = m_db->get_block_cumulative_difficulty(m_db->height() - 1);
   m_db->block_txn_stop();
   return true;
 }

src/cryptonote_core/blockchain.h

@@ -826,6 +826,16 @@ namespace cryptonote
      */
     bool for_all_outputs(std::function<bool(uint64_t amount, const crypto::hash &tx_hash, size_t tx_idx)>) const;
 
+    /**
+     * @brief get a reference to the BlockchainDB in use by Blockchain
+     *
+     * @return a reference to the BlockchainDB instance
+     */
+    const BlockchainDB& get_db() const
+    {
+      return *m_db;
+    }
+
     /**
      * @brief get a reference to the BlockchainDB in use by Blockchain
      *

src/cryptonote_core/cryptonote_core.cpp

@@ -1103,6 +1103,11 @@ namespace cryptonote
     return m_blockchain_storage.get_tail_id();
   }
   //-----------------------------------------------------------------------------------------------
+  difficulty_type core::get_block_cumulative_difficulty(uint64_t height) const
+  {
+    return m_blockchain_storage.get_db().get_block_cumulative_difficulty(height);
+  }
+  //-----------------------------------------------------------------------------------------------
   size_t core::get_pool_transactions_count() const
   {
     return m_mempool.get_transactions_count();

src/cryptonote_core/cryptonote_core.h

@@ -520,6 +520,13 @@ namespace cryptonote
       */
      crypto::hash get_tail_id() const;
 
+     /**
+      * @copydoc Blockchain::get_block_cumulative_difficulty
+      *
+      * @note see Blockchain::get_block_cumulative_difficulty
+      */
+     difficulty_type get_block_cumulative_difficulty(uint64_t height) const;
+
      /**
       * @copydoc Blockchain::get_random_outs_for_amounts
       *