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.
This needs more coordinated design than we can do right here, I think: my reaction would be that closing the applicationInput should not have an effect in the state where we are reading from the bypassInput, but you are arguing over there that reading alternatingly (as I propose) is not what we should do. A whiteboard would come in handy right now.