temp fix for lumi report on workloadsummary

temp fix for #5216 
Adding unique constraint and removing that conditional if should reasonably increase its performance, especially for failed jobs with plenty of lumis.
When you were debugging this TaskArchiver problem, do you think the size of the document to inject would be the biggest problem, like this line:
https://github.com/ticoann/WMCore/blob/temp_fix_lumi_update_ws/src/python/WMComponent/TaskArchiver/TaskArchiverPoller.py#L750
or loops are the culprit here?