[bisq-network/proposals] Allow seller to be offline and still get the payout tx signed for altcoins (#241)

[chimp1984]

To prevent the cases where the seller does not confirm the altcoin receipt we can add support for automated payment receipt for altcoins by lookup of blockexplorers. For XMR there is already a PR in progress.

This still require that the sellers app is online at the time when the altcoin gets confirmed. To mitigate that requirement we could add a delegation model for providing the signature for the payout tx to the buyer.

Concept:
Seller creates at take offer time his signature for the payout tx and splits it into chunks which he sends to different bonded nodes. Those nodes listen for altcoin payment confirmation of those who requested their service. Once they see the altcoin has been transferred they take the associated chunk of the signature and send it to one chosen node (selection done by deterministic selection based on the data similar like we select mediator). This selected node will send the full signature once it has received all chunks to the buyer (mailbox msg). The seller still can auto comfirm himself if he is online as well do a manual confirmation. So there are 2 levels of further redundancy of the system would fail (e.g. a msg does not arrive).

Security and privacy issues:
Those nodes learn about the onion addressed of both traders as well as the altcoin address and the deposit tx (trade amount). If all those nodes collude or if they get hacked the btc in the trade can be stolen without the altcoin transfer. There will be also a threshold of a subset of signatures which allow a successful brute force attack. There might be smarter schemes to split the signature (not sure if Schamir helps here). 

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https://github.com/bisq-network/proposals/issues/241
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