Implement the futures Stream trait for subscriptions

[lulf]

Instead of having a receive_message() method on the client, have the subscribe_* methods return a type holding a mut reference to the client, and implementing the Stream trait from the futures crate. Usage

let sub = client.subscribe_to_topic(...).await;

loop {
   if let Ok(message) = sub.next().await {
       // Process message
   }
}

Then when sub goes out of scope, the client can be reused.