Add ChannelMonitor::get_relevant_txids

Define an Electrum-friendly interface for ChannelMonitor where txids of
relevant transactions can be obtained. For any of these transactions
that are re-orged out of the chain, users must call
transaction_unconfirmed.

lightning/src/chain/channelmonitor.rs

@@ -1385,6 +1385,19 @@ impl<Signer: Sign> ChannelMonitor<Signer> {
 		self.inner.lock().unwrap().update_best_block(
 			header, height, broadcaster, fee_estimator, logger)
 	}
+
+	/// Returns the set of txids that should be monitored for re-organization out of the chain.
+	pub fn get_relevant_txids(&self) -> Vec<Txid> {
+		let inner = self.inner.lock().unwrap();
+		let mut txids: Vec<Txid> = inner.onchain_events_waiting_threshold_conf
+			.iter()
+			.map(|entry| entry.txid)
+			.chain(inner.onchain_tx_handler.get_relevant_txids().into_iter())
+			.collect();
+		txids.sort_unstable();
+		txids.dedup();
+		txids
+	}
 }
 
 impl<Signer: Sign> ChannelMonitorImpl<Signer> {

lightning/src/ln/onchaintx.rs

@@ -945,6 +945,16 @@ impl<ChannelSigner: Sign> OnchainTxHandler<ChannelSigner> {
 		}
 	}
 
+	pub(crate) fn get_relevant_txids(&self) -> Vec<Txid> {
+		let mut txids: Vec<Txid> = self.onchain_events_waiting_threshold_conf
+			.iter()
+			.map(|entry| entry.txid)
+			.collect();
+		txids.sort_unstable();
+		txids.dedup();
+		txids
+	}
+
 	pub(crate) fn provide_latest_holder_tx(&mut self, tx: HolderCommitmentTransaction) {
 		self.prev_holder_commitment = Some(replace(&mut self.holder_commitment, tx));
 		self.holder_htlc_sigs = None;