!htc #15726: Akka HTTP with the new DSL

/cc @jrudolph @sirthias 

TODO:
 - [X] Port http-core
 - [ ] Port rest of http
 - [ ] Port to the new IO abstraction
 - [ ] get rid of unnecessary materializer parameters
That is the goal yes. We don't want people to arrive to SPI methods via code completion easily. If this turns out to be a real issue we can add the sugar. headFuture is different tough, I am in favor of that one.