private void completeExceptionally(Throwable error) {
      if (finished) {
        return;
      }
      finished = true;
      future.completeExceptionally(error);
    }
This client class is for invoking the aggregate functions deployed on the Region Server side via
the AggregateService. This class will implement the supporting functionality for
summing/processing the individual results obtained from the AggregateService for each region.
hbase-endpoint/src/main/java/org/apache/hadoop/hbase/client/coprocessor/AsyncAggregationClient.java