various changes to runtime checkpoint updating

json checkpoints will be checked every 10 minutes, dns every 60.

json checkpoints always enforced, dns still with flag.

conflicting checkpoints is hard fail, but soft if dns enforce flag not
set and dns checkpoints are wonky.

src/cryptonote_core/checkpoints.cpp

@@ -113,4 +113,15 @@ namespace cryptonote
     return m_points;
   }
 
+  bool checkpoints::check_for_conflicts(checkpoints& other)
+  {
+    for (auto& pt : other.get_points())
+    {
+      if (m_points.count(pt.first))
+      {
+        CHECK_AND_ASSERT_MES(pt.second == m_points[pt.first], false, "Checkpoint at given height already exists, and hash for new checkpoint was different!");
+      }
+    }
+    return true;
+  }
 }