Remove SyncStream

To make a stream `Send`, we have a wrapper class which implements `Stream`.

```rust
struct SyncStream<S> {
    st: sync_wrapper::SyncWrapper<S>,
}
```

I've given back this implementation to the upstream in the following commit

https://github.com/Actyx/sync_wrapper/commit/778cffe43eff21bed80193e122178daed90e7ad0

When the new crate is out, let's use it.