Event may be more efficient than looping through all TX

[clesaege]

https://github.com/kleros/kleros-interaction/blob/master/contracts/standard/arbitration/MultipleArbitrableTransaction.sol#L358
https://github.com/kleros/kleros-interaction/blob/master/contracts/standard/arbitration/MultipleArbitrableTokenTransaction.sol#L382

For MultipleArbitrableTransaction and MultipleArbitrableTokenTransaction, the frontend is looping though all TX. It may be more efficient to use events indexed by both sender and receiver address.