Certify and tabulate completed trades

[woodser]

This issue requests the ability for the arbitrator to certify a completed trade, thus allowing traders to verify and tabulate past trades to establish peer reputability.

When a trade is completed, both peers should request the arbitrator to sign a message which certifies the completed trade amount, payment method, and probably specific payment account.

Takers may use these certifications to tabulate past completed trades to establish the maker's reputability before taking their offer.

This functionality is intended to replace Bisq's method of signing accounts and may solve #120.

How to Implement

When traders complete a trade between themselves, they should notify the arbitrator and request a signature for the completed trade.

If the arbitrator resolves a dispute, they may provide certifications for neither, one, or both traders depending on the dispute circumstances.

To verify a completed trade, the arbitrator must receive updated multisig info from a trader to update the multisig wallet and confirm withdraw from the wallet.

Currently the last message sent from the traders to the arbitrator for a non-disputed trade is DepositRequest, so a new message needs to be sent when the trade completes to notify the arbitrator for them to automatically verify and certify trade completion.