Implement request cancellation short-circuiting in juliet

Right now, the juliet crate's topmost layer (RPC) does not efficiently process request cancellations, it will simply let the response run its course and use it, instead of sending a response cancellation immediately instead, and internally discarding the response, should there be any. Instead, we would like to

- confirm request cancellations as early as possible by sending a response cancellation,
- silently discard responses if they are sent after receiving a cancellation,
- allow clients to check if a request has been cancelled if they have not yet started processing it, and
- ensure there is no attack vector by alternating request/cancel.