Both `RestServlet`s and `BaseFederationServlet`s register their handlers
with `HttpServer.register_paths` / `JsonResource.register_paths`. Update
`JsonResource` to respect the `@cancellable` flag on handlers registered
in this way.

Although `ReplicationEndpoint` also registers itself using
`register_paths`, it does not pass the handler method that would have the
`@cancellable` flag directly, and so needs separate handling.

Signed-off-by: Sean Quah <seanq@element.io>