=str #15608 Make sure ExposedPublisher is first
Merge pull request #15703 from akka/wip-15608-exposed-publisher-race-patriknw

=str #15608 Make sure ExposedPublisher is first
!str #15564 Add stream supervisor

* this is the first step, to enable safe FlowMaterializer that can be shared
* had to remove Log because of missing context (we need another solution for log)
* naming of the actors have changed, should be revisited