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.
Yes, true.
One fix would be to recover from `Http.StreamException` error coming in from the application before the `bypassMerge`. Do you see a better solution?