There are malicious behaviours that are detectable but not provable only with the gossip graph.
These will require an intervention from routing.
To allow this, we need to change the parsec API:
handle_request returns a Result<Response<_>, Error>>
handle_response` return a `Result<(), Error>
And define new enum invariant:
On receiving these, routing can communicate this to other nodes through backchannels such as: