Add a time delay to the challenge payment

[norswap]

This has to do with enabling manual intervention in case of a software bug.

If an incorrect, but unchallengeable output root is posted, governance will have 7 days to act and update the system.

However, if a correct but challengeable output root is posted, the current (unimplemented) design implies that a challenger will be able to claim the proposer bond, even though the proposer was honest and it's the system that was buggy. To avoid this, we would add a time delay (probably 7 days too) so that governance can take appropriate action.

[protolambda]

Closing as out-of-scope, Cannon focuses now on just serving the VM part of the fault proof: generating and verifying witness data. The Dispute Game continues as a separate project.