Cancellation signal is not propagated from transports to reactive controller methods

[bclozel]

As raised in #450, cancel signals are not propagated from the transport layer, through the ExecutionGraphQlService, up to the data fetchers provided by the application. This mostly applies to controller handlers with a Publisher-like return type and subscriptions in general. This is due to a well-known CompletableFuture behavior.

We can work around this limitation by propagating this signal out of band through a Sink in the GraphQL context.