int numResidentSends() {
        int count = 0;
        if (current != null)
            count += 1;
        count += sendQueue.size();
        return count;
    }
Construct a MultiRecordsSend from a queue of Send objects. The queue will be consumed as the MultiRecordsSend
progresses (on completion, it will be empty).
clients/src/main/java/org/apache/kafka/common/record/MultiRecordsSend.java