QHttpServer: Handle both HTTP 1.1 and HTTP 2.0 connections

When is QTBUG-122312 is done the HTTP 2 handshake is handled, and when QTBUG-124866 is done we should have a common user API QHttpResponder to handle HTTP 1.1 and HTTP 2 connections.

Now we need to use the QHttp2Connection and QHttp2Stream classes from QtNetwork to handle HTTP 2 connections.

The handlers for HTTP 1.1 and 2 should inherit the same interface and implement the same methods. The shared interface is used by QHttpResponder to send messages back to the client. The new framework must handle multiple simultaneous requests from the client.