SLING-2024 Apply slightly modified patch by Chetan Mehrotra (thanks alot)

* change is to refactor into o.a.s.commons.log.logback
* Removed old SLF4J implementation

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1518083 13f79535-47bb-0310-9956-ffa450edef68