HTTP/2 close behaviour is problematic on unclean close

If a HTTP/2 connection is closed by the remote client (or RST_STREAM is sent), then the completion listener is currently directly invoked by the IO thread.

This is problematic as the request might be 'owned' by a different thread, which means that the exchange state can change while the other thread is working on it.

This is particularly problematic for the reverse proxy, as if the exchange is ended the connection is returned to the pool, even though it is still processing. Another request can then get allocating this connection, and the request will need to wait until the first backend request has finished.