[SPARK-19900][CORE] Remove driver when relaunching.

This is https://github.com/apache/spark/pull/17888 .

Below are some spark ui snapshots.

Master, after worker disconnects:

<img width="1433" alt="master_disconnect" src="https://cloud.githubusercontent.com/assets/2576762/26398687/d0ee228e-40ac-11e7-986d-d3b57b87029f.png">

Master, after worker reconnects, notice the `running drivers` part:

<img width="1412" alt="master_reconnects" src="https://cloud.githubusercontent.com/assets/2576762/26398697/d50735a4-40ac-11e7-80d8-6e9e1cf0b62f.png">

This patch, after worker disconnects:
<img width="1412" alt="patch_disconnect" src="https://cloud.githubusercontent.com/assets/2576762/26398009/c015d3dc-40aa-11e7-8bb4-df11a1f66645.png">

This patch, after worker reconnects:
![image](https://cloud.githubusercontent.com/assets/2576762/26398037/d313769c-40aa-11e7-8613-5f157d193150.png)

cc cloud-fan jiangxb1987

Author: Li Yichao <lyc@zhihu.com>

Closes #18084 from liyichao/SPARK-19900-1.