Made sctp PollStream Send + Sync

[emcell]

I've tried using sctp in a context outside of webrtc and tried the following.

use std::sync::Arc;

use tokio::{io::AsyncReadExt, net::UdpSocket};
use webrtc_sctp::{
    association::{Association, Config},
    chunk::chunk_payload_data::PayloadProtocolIdentifier,
    stream::PollStream,
};

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let conn = Arc::new(UdpSocket::bind("0.0.0.0:0").await.unwrap());
    conn.connect("127.0.0.1:132").await.unwrap();

    let config = Config {
        net_conn: conn,
        max_receive_buffer_size: 0,
        max_message_size: 0,
        name: "client".to_owned(),
    };
    let association = Association::client(config).await?;

    let stream = association
        .open_stream(0, PayloadProtocolIdentifier::Binary)
        .await?;
    let stream = PollStream::new(stream);
    tokio::spawn(async move {
        let mut buf = [0u8; 10];
        stream.read(&mut buf).await;
    });

    Ok(())
}

This code wasn't compiling because PollStream is not Send+Sync.

This PR fixes the problem. I think this doesn't cause any Problems.

What do you think?