Align cancelling behavior to graphql-java request execution

[bclozel]

As of graphql-java/graphql-java#3879, cancellation is natively supported by graphql-java at the ExecutionInput level.
We can now retire our custom wrapping of DataFetchers for that and only propagate the cancellation signal from the transport layer to the ExecutionInput#cancel().

[bclozel]

The graphql-java feature is delayed, let's reschedule this for a future release.