Orderbook chain-commitment: simple `OP_RETURN` flow

[ariard]

I looked more on the submitToMainstay flow.

For now, I think we can do something far simpler for the first releases.

Every 24 hours, take all the orderId from the latest 24 hours.

Concatenate them in a single data byte vector sorted by alphanumeric order.

Hash the vector with an hashing function (e.g SHA256 or SipHash).

Commit the 32-byte hash output in an OP_RETURN bitcoin tx output scriptpubkey.

Broadcast and confirm the bitcoin tx.