[SPARK-20966][WEB-UI][SQL] Table data is not sorted by startTime time desc,...

[SPARK-20966][WEB-UI][SQL] Table data is not sorted by startTime time desc, time is not formatted and redundant code in JDBC/ODBC Server page.

## What changes were proposed in this pull request?

1. Question 1 : Table data is not sorted by startTime time desc in JDBC/ODBC Server page.

fix before :
![2](https://cloud.githubusercontent.com/assets/26266482/26718483/bf4a0fa8-47b3-11e7-9a27-dc6a67165b16.png)

fix after :
![21](https://cloud.githubusercontent.com/assets/26266482/26718544/eb7376c8-47b3-11e7-9117-1bc68dfec92c.png)

2. Question 2 :  time is not formatted in JDBC/ODBC Server page.

fix before :
![1](https://cloud.githubusercontent.com/assets/26266482/26718573/0497d86a-47b4-11e7-945b-582aaa103949.png)

fix after :
![11](https://cloud.githubusercontent.com/assets/26266482/26718602/21371ad0-47b4-11e7-9587-c5114d10ab2c.png)

3. Question 3 :  Redundant code in the ThriftServerSessionPage.scala.
The function of 'generateSessionStatsTable'  has not been used

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes #18186 from guoxiaolongzte/SPARK-20966.