=str #16040 Use scaladsl2 in TCK tests

* found and fixed one issue: onError was invoked twice for the overflow
  error
* and also noticed that we restart the actors, instead of stopping them
  in case of internal error, changed to stopping strategy
Merge pull request #16044 from akka/wip-16040-tck-scaladsl2-patriknw

=str #16040 Use scaladsl2 in TCK tests