`ServerBinding` termination does not close HTTP/2 connections with h2c Upgrade connection setup

Calling the `ServerBinding` `terminate()` method appears to not close open HTTP/2 connections reused by a client for different requests, even after the `hardDeadline` timeout is exceeded. In fact, **new requests are still handled successfully** even after the termination future completes.

Steps to reproduce (in 1.1.0):
1. Use `enable-http2=on` in the Typesafe Config.
2. Create a binding with `Http().newServerAt(...).bind(...)`.
3. Make an HTTP request to this service with a client. I used [Unirest](https://kong.github.io/unirest-java/) for this.
4. Request binding termination and await for it.
5. Make more HTTP requests with the same connection. [Unirest](https://kong.github.io/unirest-java/) does that behind the scenes.
6. Verify that the service still responds, even though it was supposed to be terminated.

Start by reproducing the issue with enable-http2=on, Http().newServerAt(...).bind(...), and a connection-reusing Unirest client. Then inspect the ServerBinding terminate() path and h2c Upgrade connection handling. Done means termination closes reused HTTP/2 connections after the hardDeadline and later requests are no longer handled.

Written by the indexing model from the issue text.