message: nydusd: increase default fscache threads
  Only one service thread can be a performance bottleneck for fscache
  mode, increase the default fscache threads to 4.
diff: @@ -343,7 +343,7 @@ fn append_services_subcmd_options(app: App<'static, 'static>) -> App<'static, 's
  .arg(
  Arg::with_name("fscache-threads")
  .long("fscache-threads")
  - .default_value("1")
  + .default_value("4")
  .help("Number of working threads to serve fscache requests")
  .takes_value(true)
  .required(false)
  