Feat: updated lastInteraction when task is resolved

[hbarcelos]

Motivation

Currently there is a problem to identify exactly when a task is marked as resolved.

So far I identified the following use case:

• When displaying resolved tasks in the UI, we should sort them by descending completion date to give a feel of 'liveness'.

There are probably more though.

Risk Analysis

Regarding the risks introduced by such changes: all methods that perform some sort of verification on lastInteraction:

• assignTask

  kleros-interaction/contracts/standard/arbitration/Linguo.sol

  Line 215 in 0e27cc7

  function assignTask(uint _taskID) external payable {

• submitTranslation

  kleros-interaction/contracts/standard/arbitration/Linguo.sol

  Line 243 in 0e27cc7

  function submitTranslation(uint _taskID, string _translation) external {

• reimburseRequester

  kleros-interaction/contracts/standard/arbitration/Linguo.sol

  Line 257 in 0e27cc7

  function reimburseRequester(uint _taskID) external {

• challengeTranslation

  kleros-interaction/contracts/standard/arbitration/Linguo.sol

  Line 289 in 0e27cc7

  function challengeTranslation(uint _taskID) external payable {

All of them:

• Have an additional status guard.
• Change status if all conditions are met. This means other transactions that depend on the previous lastInteraction would not be allowed anyway.

Cost Analysis

Marginal cost for this change would be as high as 20,000 gas (SSTORE gas spending according to this).

[hbarcelos]

Withdrawing in favor of #310.