Delay cosign acknowledgement by an explicit time delay

[kayabaNerve]

Per #339, we panic upon seeing cosigns to distinct chains (where each cosign has 17% of the total stake). This assume that if two 67% cosigns exist, they successfully propagate across the network before any sign protocol completes. We should explicitly add a time delay for this propagation latency (even though it should be less than the sign protocol time).

So long as we only delay cosign acknowledgement, not production, this should be a fixed delay (not an ever-growing one).