=str #15755 #15756 source and sink

sorry, deleted the old branch by accident, previous discussion was at #15786 
Okay, I thought long and hard and there is no solution.

Luckily I found one nevertheless, stay tuned ;-) (basically we need to demand that the materializer used for the flow knows how to materialize the given Source and Sink, with two prescribed ways used by Akka itself)