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.
`isFinishing` appears to signal that `absorbTermination` has been called.
Can I call `absorbTermination` also during normal operation? Is it guaranteed that I won't receive an `onPush` call anymore afterwards?