We'll need to implement some reasonable timeouts in our SDKs, in order to prevent requests from hanging forever due to misbehavior on the remote side, but when a timeout occurs, we ideally want some way to signal that as well.

This PR introduces the concept of a cancellation notification, issued to indicate that a previous request (one that SHOULD still be in-flight) is being cancelled by its issuer and the result will be ignored. The recipient can use this to interrupt any processing that's still happening, if applicable.

Resolves #10 (I think by being a better solution than timeouts defined in messages).