`BaseFederationServlet` wraps its endpoints in a bunch of async code
that has not been vetted for compatibility with cancellation.
Fail CI if a `@cancellable` flag is applied to a federation endpoint.
Signed-off-by: Sean Quah <seanq@element.io>
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>
