message: Remove Sync requirement for the handler function used with a Subscription
diff: @@ -159,7 +159,7 @@ impl Subscription {
  /// ```
  pub fn with_handler<F>(self, handler: F) -> Handler
  where
  - F: Fn(Message) -> io::Result<()> + Sync + Send + 'static,
  + F: Fn(Message) -> io::Result<()> + Send + 'static,
  {
  // This will allow us to not have to capture the return. When it is dropped it
  // will not unsubscribe from the server.
  