Cancel is not safety

[gaoqiangz]

let mut client = Client::connect(config, tcp.compat_write()).await?;

//cancel while executing
{
let rv = tokio::time::timeout(
    std::time::Duration::from_secs(3),
    client.simple_query("WAITFOR DELAY  '00:10'"),
)
.await;
println!("{:?}", rv);
}

//will never complete
{
let rv = client.simple_query("SELECT 'never complete'").await;
println!("{:?}", rv);
}