The ExecutorClassLoader for REPL will cause Janino failed to find class for those in java.lang, so switch to use default class loader for Janino, which will also help performance.

cc liancheng yhuai

Author: Davies Liu <davies@databricks.com>

Closes #6898 from davies/fix_class_loader and squashes the following commits:

24276d4 [Davies Liu] add regression test
4ff0457 [Davies Liu] address comment, refactor
7f5ffbe [Davies Liu] fix REPL class loader with codegen