[SPARK-18646][REPL] Set parent classloader as null for ExecutorClassLoader

## What changes were proposed in this pull request?

`ClassLoader` will preferentially load class from `parent`. Only when `parent` is null or the load failed, that it will call the overridden `findClass` function. To avoid the potential issue caused by loading class using inappropriate class loader, we should set the `parent` of `ClassLoader` to null, so that we can fully control which class loader is used.

This is take over of #17074,  the primary author of this PR is taroplus .

Should close #17074 after this PR get merged.

## How was this patch tested?

Add test case in `ExecutorClassLoaderSuite`.

Author: Kohki Nishio <taroplus@me.com>
Author: Xingbo Jiang <xingbo.jiang@databricks.com>

Closes #18614 from jiangxb1987/executor_classloader.