Realitio.submitAnswerCommitment should enforce a minimum commitment timeout

[pacamara]

Realitio.submitAnswerCommitment assigns a commitment timeout of 1/8th of the question timeout.

Therefore for a high frequency application of Realitio with question timeout 2 minutes (plausible and not excluded by the docs), the commitment timeout is within the average block time. This behaviour is not documented. If interacting with Realitio via the Kleros RealitioArbitratorProxy, this enables a malicious miner to time out any commitment and claim the bounty.

Suggest Realitio.submitAnswerCommitment uses require(...) to enforce a minimum commitment timeout which is a safe multiple of the average block time. Or alternatively, commitment timeout/question timeout ratio is increased for short question timeouts.

See pacamara#1 for demonstration code.

(Issue identified from code review for bounty kleros/kleros-interaction#244 )