code: synchronized public void completeTask(int taskId, int partitionStoresMigrated) {
          tasksInFlight.remove(taskId);
  
          numTasksCompleted++;
          numPartitionStoresMigrated += partitionStoresMigrated;
  
          updateProgressBar();
      }
docs: Called whenever a rebalance task completes. This means one task is done
  and some number of partition stores have been migrated.
  
  @param taskId
  @param partitionStoresMigrated Number of partition stores moved by this
  completed task.