Find unlock tx hash matching a used transfer finalization proof

[paouvrard]

We can use eth_getStorageAt to check that a proof was used i.e that the transfer has already been finalized: aurora-is-near/rainbow-bridge-client#37

This works well for checking that a proof was used, but the event emited by unlockToken https://github.com/aurora-is-near/rainbow-token-connector/blob/23b0c3aca6c5755b70cb6117d20c12e9a640fc37/erc20-connector/contracts/ERC20Locker.sol#L67 doesn't allow us to uniquely identify the unlock tx hash which used the proof.

The rainbow-bridge-frontend currently doesn't do this but we would want users to have an etherscan link to the tx which finalized the transfer. Aurora webapp already has the finalization tx hash available.

This feature is only necessary for transfers which a recovered. For regular transfer we know the finalization tx hash since it is broadcasted by the user.