Refactored how all requests are handled by VaadinServlet and VaadinPortlet (#11192)

Refactored how all requests are handled by VaadinServlet and VaadinPortlet (#11192)

* Handling is now based on a list of RequestHandlers in VaadinService
* Request handling logic has been moved to VaadinService
* Users can customize the list by adding own (service level) request handlers
* For users specific request handlers you can still use the request handlers in VaadinSession
* Deprecated RequestType - all handlers are given the opportunity to handle a request until one of them chooses to handle it. RequestType makes no sense as it does not tell which handler will handle the request.
* Removed serveStaticResource which has never been used

Change-Id: Ia7d088535e46430ca8adf631d3f1dd944b9d51e2