Add more checks around the Hadoop modified classes we use.
- some dev docs (package level javadocs?) explaining the whole org.apache.hadoop "shadow" class impls
- ideally include some hook in HdfsTestUtil that causes it to fail fast if it can detect that the classloader didn't pick up our modified classes? .. maybe by adding a new public static final Object SOLR_HACK_FOR_CLASS_VERIFICATION = new Object to each org.apache.hadoop.** that it could check for via reflection?