Unique barriers for each round in StressSpec.exerciseSupervision, see #2905

* Looks like the issue was caused by a mixup of barriers and/or name
  of the result aggregator actor
Merge pull request #1032 from akka/wip-2905-StressSpec-exercise-supervision-patriknw

Unique barriers for each round in StressSpec.exerciseSupervision, see #2905