Update to 1.821
    
    - New upstream release 1.821
      - Improved reliability when running MCE with threads
      - Added parallel Net::Pcap and Ping demonstrations on Github:
        https://github.com/marioroy/mce-examples/tree/master/network
      - Optimized 'dequeue' method in MCE::Queue
      - Optimized 'synchronize' method in MCE::Mutex