ADR for DLC collaborative settlement

[luckysori]

We need to define what happens when a DLC in a DLC channel is settled off-chain. This is a requirement, as we don't want to go on-chain every time that a CFD is closed. rust-dlc doesn't support settling the DLC output into the channel; it only supports collaborative settlement on-chain.

I can see 2 choices:

1. DLC is replaced by 1-2 outputs in the DLC channel paying directly to either party.
2. DLC channel is removed from the split_transaction and the coins are allocated to outputs paying to either party in the Lightning commitment transaction.

Option 1 requires modifying regular rust-dlc to support outputs other than a DLC in the buffer_transaction. Interestingly, this option means that the non-DLC funds in the DLC channel would only be readily available for trading.

[Preferred] Option 2 requires investigating the feasibility of going back and forth between a split_transaction and a regular funding_transaction as the user opens and closes positions.

Let's document our choice in an ADR and create a follow-up ticket with further steps towards the implementation.

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Having discussed the product situation with @da-kami, we reached the conclusion that we want the funds in the sibling channels to either be (1) in a DLC or (2) readily available for Ligthning payments. This means that we've eliminated the concept of the trading wallet.

Given this, it appears to make little sense to go for option 1, as we don't want to write code to have coins sitting around in a hidden "trading wallet".

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