[SPARK-16563][SQL] fix spark sql thrift server FetchResults bug

## What changes were proposed in this pull request?

Add a constant iterator which point to head of result. The header will be used to reset iterator when fetch result from first row repeatedly.
JIRA ticket https://issues.apache.org/jira/browse/SPARK-16563

## How was this patch tested?

This bug was found when using Cloudera HUE connecting to spark sql thrift server, currently SQL statement result can be only fetched for once. The fix was tested manually with Cloudera HUE, With this fix, HUE can fetch spark SQL results repeatedly through thrift server.

Author: Alice <alice.gugu@gmail.com>
Author: Alice <guhq@garena.com>

Closes #14218 from alicegugu/SparkSQLFetchResultsBug.