A slave can't preprocess two tasks.

[taeguk]

When a slave preprocess one task, it can't preprocess another task parrallely.
This issue is basically caused by #18.

The solutions

1. Fix Communication with Cloud DFS is a big bottleneck. #18. (Can process two messages parrallely. But it requires many changes of codes and design.)
2. Use a thread or process for preprocessing a task. (This is simple relatively)