message: nydusd: enlarge default fuse server threads
  Now the default value is only 1, it affacts performance.
diff: @@ -263,7 +263,7 @@ fn append_fuse_options(app: App<'static, 'static>) -> App<'static, 'static> {
  Arg::with_name("threads")
  .long("thread-num")
  .short("T")
  - .default_value("1")
  + .default_value("4")
  .help("Number of worker threads to serve IO requests")
  .takes_value(true)
  .required(false)
  