[BUG] Mars web got stuck when chunk graph is large

**Describe the bug**
After a tileable graph with small chunk size is submitted and after a while, Mars Web got stuck.

**To Reproduce**
Use code below to reproduce:

```python
import mars
import mars.tensor as mt
sess = mars.new_session('http://localhost:13042/')
a = mt.ones((1000, 1000), chunk_size=10)
b = (a + 1) * 2 + (a - 3) ** 2
b.execute()
```
