Add server-side support for `Expect: 100-continue`, other improvements

This PR improves a number of things in akka-http-core and adds server-side support for `Expect: 100-continue` (#15799).

The client-side support depends on #16336 and is therefore postponed for the time being.
We *set* `terminated = true` after having parsed the end of a request, which might be well before the last thing is being consumed in `onPull`. If I want to get rid of the `terminated` flag I'd have to call `absorbTermination` at the time I currently set `terminated = true`, which is not within an `onUpstreamXXX` call.