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.h

@@ -47,6 +47,7 @@ namespace cryptonote
     bool is_alternative_block_allowed(uint64_t blockchain_height, uint64_t block_height) const;
     uint64_t get_max_height();
     const std::map<uint64_t, crypto::hash>& get_points();
+    bool check_for_conflicts(checkpoints& other);
   private:
     std::map<uint64_t, crypto::hash> m_points;
   };