daemon: guard against rare 'difficulty drift' bug with checkpoints and recalculation

On startup, it checks against the difficulty checkpoints, and if any mismatch is found, recalculates all the blocks with wrong difficulties. Additionally, once a week it recalculates difficulties of blocks after the last difficulty checkpoint.

src/cryptonote_core/cryptonote_core.cpp

@@ -1662,6 +1662,7 @@ namespace cryptonote
     m_check_disk_space_interval.do_call(boost::bind(&core::check_disk_space, this));
     m_block_rate_interval.do_call(boost::bind(&core::check_block_rate, this));
     m_blockchain_pruning_interval.do_call(boost::bind(&core::update_blockchain_pruning, this));
+    m_diff_recalc_interval.do_call(boost::bind(&core::recalculate_difficulties, this));
     m_miner.on_idle();
     m_mempool.on_idle();
     return true;
@@ -1923,6 +1924,12 @@ namespace cryptonote
     return true;
   }
   //-----------------------------------------------------------------------------------------------
+  bool core::recalculate_difficulties()
+  {
+    m_blockchain_storage.recalculate_difficulties();
+    return true;
+  }
+  //-----------------------------------------------------------------------------------------------
   void core::flush_bad_txs_cache()
   {
     bad_semantics_txes_lock.lock();