Improve performance

1. add method encodeMsg2, Improve performance 1x
2. optimize performance encodeMsg
3. optimize performance decodeMsg, Improve performance 1x
4. change msg construct add params opts
   • opts, {useIconv: true }
     • useIconv, boolean, true(default): use iconv-lite convert code.( false, Improve performance)
5. Default string code,change to utf8