!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, I see.
We use "upstream of X" also as a label for the publisher that X is currently attached to, or rather: the subscription to this publisher. So "cancelling one's upstream" does indeed make sense to me.
But "subscription" would of course be more correct.