Improve ExecutorMonitor shuffle-tracking to handle speculative tasks properly

// If the task succeeded and the stage generates shuffle data, record that this executor
// holds data for the shuffle. This code will track all executors that generate shuffle
// for the stage, even if speculative tasks generate duplicate shuffle data and end up
// being ignored by the map output tracker.
//
// This means that an executor may be marked as having shuffle data, and thus prevented
// from being removed, even though the data may not be used.
if (shuffleTrackingEnabled && event.reason == Success) {
  stageToShuffleID.get(event.stageId).foreach { shuffleId =>
    exec.addShuffle(shuffleId)
  }
}