no-jira : adding WithholdingAppender which entirely muted log output as long everything is fine, and instead writes the withheld (ie muted) log output in case of a failure - to reduce log output of ClusterLoadTest in success cases

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1573182 13f79535-47bb-0310-9956-ffa450edef68