Small rewrite in JobManager.java to fix a bug where jobs could be duplic...

...ated under certain conditions. https://issues.apache.org/jira/browse/OFBIZ-5496

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/branches/release12.04@1559346 13f79535-47bb-0310-9956-ffa450edef68