Consumer should ensure consistent ordering between sync and async offset commits

If commitSync is called with an async offset commit in-flight, it is possible (even likely) for it to return prior to the invocation of the async callback even though the OffsetCommit request for the latter will have actually completed first. In this case, the async callback is not invoked until the next call to poll, which can lead to very confusing behavior since the client is led to think that the async commit happened second.