SLING-3341 - [logback] Race condition in configuring Logback

Making the configChange check more robust against race condition. The check is now performed under a lock such that change in flag does not get missed out in any case

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1561666 13f79535-47bb-0310-9956-ffa450edef68