Use Loader.loadSystemClass

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1587432 13f79535-47bb-0310-9956-ffa450edef68
Use Loader.newCheckedInstanceOf

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1587433 13f79535-47bb-0310-9956-ffa450edef68