void onComplete() {
            for (Runnable onComplete : completionHandlers) {
                onComplete.run();
            }
        }
Handles new generated assignments, to be propagated to the controller.
Assignment events may be handled out of order, so for any two assignment
events for the same topic partition, the one with the oldest timestamp is
disregarded.
server/src/main/java/org/apache/kafka/server/AssignmentsManager.java