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.
HTTP/2.0 is not yet supported by this model or anything else in akka-http.
This is not the only place that will need to be touched when we add HTTP/2.0 support.
I can certainly turn things around here but it won't really make a difference, now or later.