Move update_checkpoints() to a later stage

update_checkpoints() makes a few DNS requests and can take up to 20-30 seconds to complete (3-6 seconds on average). It is currently called from core::handle_incoming_block() which holds m_incoming_tx_lock, so it blocks all incoming transactions and blocks processing while update_checkpoints() is running. This PR moves it to until after a new block has been processed and relayed, to avoid full monerod locking.

src/cryptonote_core/cryptonote_core.cpp

@@ -1656,10 +1656,6 @@ namespace cryptonote
     if (((size_t)-1) <= 0xffffffff && block_blob.size() >= 0x3fffffff)
       MWARNING("This block's size is " << block_blob.size() << ", closing on the 32 bit limit");
 
-    // load json & DNS checkpoints every 10min/hour respectively,
-    // and verify them with respect to what blocks we already have
-    CHECK_AND_ASSERT_MES(update_checkpoints(), false, "One or more checkpoints loaded from json or dns conflicted with existing checkpoints.");
-
     block lb;
     if (!b)
     {

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -537,6 +537,10 @@ namespace cryptonote
       MLOG_PEER_STATE("requesting chain");
     }
 
+    // load json & DNS checkpoints every 10min/hour respectively,
+    // and verify them with respect to what blocks we already have
+    CHECK_AND_ASSERT_MES(m_core.update_checkpoints(), 1, "One or more checkpoints loaded from json or dns conflicted with existing checkpoints.");
+
     return 1;
   }
   //------------------------------------------------------------------------------------------------------------------------
@@ -819,6 +823,10 @@ namespace cryptonote
           post_notify<NOTIFY_REQUEST_CHAIN>(r, context);
           MLOG_PEER_STATE("requesting chain");
         }            
+
+        // load json & DNS checkpoints every 10min/hour respectively,
+        // and verify them with respect to what blocks we already have
+        CHECK_AND_ASSERT_MES(m_core.update_checkpoints(), 1, "One or more checkpoints loaded from json or dns conflicted with existing checkpoints.");
       }
     } 
     else