add an example of producer batching usage

[Geal]

I just implemented message batching in TopicProducer in 182d450 and I have found that using it can be a bit tricky. calling send() on a prducer returns a future of CommandSendReceipt. When batching, that future will not resolved until the batch has been sent, so we cannot do producer.send(...).await in a loop, as is done in the round trip example, because we would be stuck at the first send() https://github.com/wyyerd/pulsar-rs/blob/182d45071c02e6f5885dcfa8293bd2a299178d6c/examples/round_trip.rs#L74-L81
Instead, we need to collect the receipt futures, and await on them all at once:

let producer = pulsar.create_producer(
            "test",
            Some("my-producer".to_string()),
            producer::ProducerOptions {
                batch_size: Some(5),
                ..Default::default()
            }).await.unwrap();

let mut receipts = Vec::new();
        loop {
            let receipt = async {
                producer.send(
                    TestData {
                        data: "data".to_string(),
                    },
                    ).await
            };
            receipts.push(receipt);
            //.await.unwrap();
            counter += 1;
            if counter % 5 == 0 {
                println!("sent {} messages", counter);
                break;
            }
        }

        println!("received receipts: {:?}",join_all(receipts).await);

Maybe there's a better way to represent this in the API?

[Geal]

done in 81f0bbb after changing the message sending API in 024dd08