title: Netty ByteToMessageDecoder can't be @sharable
body: <p>I use <code>MyDecoder</code> which extends <code>ByteToMessageDecoder</code> to get Message from the socket Stream.
  It works fine in one thread. But in more threads, the netty has reported 'the handler should be sharable'
  However, I search in netty api, the <code>ByteToMessageDecoder</code> can't be @sharable, so how can I use
  it in multi thread.</p>