[SPARK-13747][SQL] Fix concurrent query with fork-join pool

## What changes were proposed in this pull request?

Fix this use case, which was already fixed in SPARK-10548 in 1.6 but was broken in master due to #9264:

```
(1 to 100).par.foreach { _ => sc.parallelize(1 to 5).map { i => (i, i) }.toDF("a", "b").count() }
```

This threw `IllegalArgumentException` consistently before this patch. For more detail, see the JIRA.

## How was this patch tested?

New test in `SQLExecutionSuite`.

Author: Andrew Or <andrew@databricks.com>

Closes #11586 from andrewor14/fix-concurrent-sql.