!str #15564 Add stream supervisor

* this is the first step, to enable safe FlowMaterializer that can be shared
  the ask-await is not the final solution
* had to remove Log because of missing context (we need another solution for log)
* naming of the actors have changed, should be revisited
* disabled tests of Zip, Concat and Merge because a race condition is exposed by
  this change, but that exists in release-2.3-dev also
more thinking has led me to believe that with `attachChild` no FIXME is needed