@Override
  public boolean mapStarted() {
    boolean b = super.mapStarted();
    return runningViaChild ? done() : b;
  }
Extension of ExecDriver:
- can optionally spawn a map-reduce task from a separate jvm
- will make last minute adjustments to map-reduce job parameters, viz:
  * estimating number of reducers
  * estimating whether job should run locally
ql/src/java/org/apache/hadoop/hive/ql/exec/mr/MapRedTask.java