!htc #15716: Akka HTTP with the new DSL

/cc @jrudolph @sirthias 

TODO:
 - [X] Port http-core
 - [x] Port rest of http
 - ~~Port to the new IO abstraction~~
 - [x] get rid of unnecessary materializer parameters
Ok, noted.
Ideally I'd like to write the Java Routing DSL (I assume that's the one you are talking about) in Scala.
Everything else will be a real pain. I guess we'll sort out potential variance issues when we get there.