Simple batch produce

[nicholastmosher]

Subtask of #847

Here is the first shot at implementing the batch producer. I have included an example and everything is working. The only thing that is still up in the air is trying to see whether we can create a more flexible API.

Currently, send_all can take any argument that can turn into an iterator over (Option<Key>, Value) pairs. This is one example of how calling it looks:

let batch: Vec<_> = (0..10)
    .map(|i| (Some(i.to_string()), format!("This is record {}", i)))
    .collect();
producer.send_all(batch).await?;

[nicholastmosher]

After a good bit of experimentation, I have not found a way to support all three calling patterns. The one that I have included in this PR is the most general one, and it is still pretty easy to use. @sehz I would like to move forward with merging this as-is, and if we want to prioritize new calling patterns in the future we can make a new issue for it.

[sehz]

rebase as well

[sehz]

LGTM