Change name of async producer parameter to "non-blocking"

[rnowling-memphis]

The producer API supports the ability to run in a blocking and non-blocking modes. In the non-blocking mode, the request is run in a green thread so that main thread isn't blocked. We currently use the term "async" for the parameter, which is confusing because it implies that we don't wait for a response. This isn't true. Change the parameter name and description to "nonblocking" or similar to clarify this to the user.