Use min mempool feerate for outbound updates on anchor channels

As done with inbound feerate updates, we can afford to commit less in
fees, as long as we still may the minimum mempool feerate. This enables
users to spend a bit more of their balance, as less funds are being
committed to transaction fees.

lightning/src/ln/channelmanager.rs

@@ -4235,13 +4235,19 @@ where
 		PersistenceNotifierGuard::optionally_notify(&self.total_consistency_lock, &self.persistence_notifier, || {
 			let mut should_persist = self.process_background_events();
 
-			let new_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::Normal);
+			let normal_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::Normal);
+			let min_mempool_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::MempoolMinimum);
 
 			let per_peer_state = self.per_peer_state.read().unwrap();
 			for (_cp_id, peer_state_mutex) in per_peer_state.iter() {
 				let mut peer_state_lock = peer_state_mutex.lock().unwrap();
 				let peer_state = &mut *peer_state_lock;
 				for (chan_id, chan) in peer_state.channel_by_id.iter_mut() {
+					let new_feerate = if chan.context.get_channel_type().supports_anchors_zero_fee_htlc_tx() {
+						min_mempool_feerate
+					} else {
+						normal_feerate
+					};
 					let chan_needs_persist = self.update_channel_fee(chan_id, chan, new_feerate);
 					if chan_needs_persist == NotifyOption::DoPersist { should_persist = NotifyOption::DoPersist; }
 				}
@@ -4271,7 +4277,8 @@ where
 		PersistenceNotifierGuard::optionally_notify(&self.total_consistency_lock, &self.persistence_notifier, || {
 			let mut should_persist = self.process_background_events();
 
-			let new_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::Normal);
+			let normal_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::Normal);
+			let min_mempool_feerate = self.fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::MempoolMinimum);
 
 			let mut handle_errors: Vec<(Result<(), _>, _)> = Vec::new();
 			let mut timed_out_mpp_htlcs = Vec::new();
@@ -4284,6 +4291,11 @@ where
 					let pending_msg_events = &mut peer_state.pending_msg_events;
 					let counterparty_node_id = *counterparty_node_id;
 					peer_state.channel_by_id.retain(|chan_id, chan| {
+						let new_feerate = if chan.context.get_channel_type().supports_anchors_zero_fee_htlc_tx() {
+							min_mempool_feerate
+						} else {
+							normal_feerate
+						};
 						let chan_needs_persist = self.update_channel_fee(chan_id, chan, new_feerate);
 						if chan_needs_persist == NotifyOption::DoPersist { should_persist = NotifyOption::DoPersist; }