`rumqttc`: `impl Sync for framed::N`?

# Failure Information (for bugs)

```
error[E0277]: `(dyn rumqttc::framed::N + 'static)` cannot be shared between threads safely
...
the trait `std::marker::Sync` is not implemented for `(dyn rumqttc::framed::N + 'static)`, which is required by `impl std::future::Future<Output = ()>: std::marker::Send`
````

