prevent user classes from loading classes in the parent classpath with ApplicationClassLoader

Today, a user class is able to trigger loading a class from Hadoop's dependencies, with or without the use of ApplicationClassLoader, and it creates an implicit dependence from users' code on Hadoop's dependencies, and as a result dependency conflicts.

We should modify ApplicationClassLoader to prevent a user class from loading a class from the parent classpath.

This should also cover resource loading (including ClassLoader.getResources() and as a corollary ServiceLoader).