Store the original funding txo in ChannelMonitor

Currently, when a ChannelMonitor is persisted, it's funding txo is used
as the key. However, when a channel is spliced, there is a new funding
txo. Store the original funding txo in ChannelMonitor such that the
persistence does not change. This will be used for v1 channels for
backward compatibility while v2 channels will use the channel ID, which
for them is not derived from the funding txo.

Author: jkczyz

lightning/src/chain/channelmonitor.rs

@@ -875,6 +875,7 @@ pub(crate) struct ChannelMonitorImpl<Signer: EcdsaChannelSigner> {
 	holder_revocation_basepoint: RevocationBasepoint,
 	channel_id: ChannelId,
 	funding_info: (OutPoint, ScriptBuf),
+	original_funding_txo: OutPoint,
 	current_counterparty_commitment_txid: Option<Txid>,
 	prev_counterparty_commitment_txid: Option<Txid>,
 
@@ -1236,6 +1237,7 @@ impl<Signer: EcdsaChannelSigner> Writeable for ChannelMonitorImpl<Signer> {
 			(21, self.balances_empty_height, option),
 			(23, self.holder_pays_commitment_tx_fee, option),
 			(25, self.payment_preimages, required),
+			(27, self.original_funding_txo, required),
 		});
 
 		Ok(())
@@ -1388,6 +1390,8 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 		let mut outputs_to_watch = new_hash_map();
 		outputs_to_watch.insert(funding_info.0.txid, vec![(funding_info.0.index as u32, funding_info.1.clone())]);
 
+		let original_funding_txo = funding_info.0;
+
 		Self::from_impl(ChannelMonitorImpl {
 			latest_update_id: 0,
 			commitment_transaction_number_obscure_factor,
@@ -1401,6 +1405,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 			holder_revocation_basepoint,
 			channel_id,
 			funding_info,
+			original_funding_txo,
 			current_counterparty_commitment_txid: None,
 			prev_counterparty_commitment_txid: None,
 
@@ -4956,6 +4961,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 		let mut channel_id = None;
 		let mut holder_pays_commitment_tx_fee = None;
 		let mut payment_preimages_with_info: Option<HashMap<_, _>> = None;
+		let mut original_funding_txo = RequiredWrapper(None);
 		read_tlv_fields!(reader, {
 			(1, funding_spend_confirmed, option),
 			(3, htlcs_resolved_on_chain, optional_vec),
@@ -4970,6 +4976,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			(21, balances_empty_height, option),
 			(23, holder_pays_commitment_tx_fee, option),
 			(25, payment_preimages_with_info, option),
+			(27, original_funding_txo, (default_value, funding_info.0)),
 		});
 		if let Some(payment_preimages_with_info) = payment_preimages_with_info {
 			if payment_preimages_with_info.len() != payment_preimages.len() {
@@ -5022,6 +5029,7 @@ impl<'a, 'b, ES: EntropySource, SP: SignerProvider> ReadableArgs<(&'a ES, &'b SP
 			holder_revocation_basepoint,
 			channel_id: channel_id.unwrap_or(ChannelId::v1_from_funding_outpoint(outpoint)),
 			funding_info,
+			original_funding_txo: original_funding_txo.0.unwrap(),
 			current_counterparty_commitment_txid,
 			prev_counterparty_commitment_txid,