MapTasks need to know whether or not they should start node-level combiner agaist outputs of mapper on their node.
The new umbilical RPC, getAggregationTargets(), is used to get outputs to be aggregated on the node. The definition as follows:
AggregationTarget getAggregationTargets(TaskAttemptID aggregator) throws IOException;
AggregationTarget is a abstraction class of array of TaskAttemptID to be aggregated.