ln: cache feerate by channel type for multi-channel updates

Add a cache by channel type for the cases where we're updating many
channels at a time, to minimize calls to the fee estimator. We can't
move this check out of the loop without duplicating the logic in
holder_preferred_commitment_sat_per_1000_weight_for_type, so we settle
for a cache to prevent duplicate queries.

Author: carlaKC

lightning/src/ln/channelmanager.rs

@@ -7223,6 +7223,7 @@ where
 	pub fn maybe_update_chan_fees(&self) {
 		PersistenceNotifierGuard::optionally_notify(self, || {
 			let mut should_persist = NotifyOption::SkipPersistNoEvents;
+			let mut feerate_cache = new_hash_map();
 
 			let per_peer_state = self.per_peer_state.read().unwrap();
 			for (_cp_id, peer_state_mutex) in per_peer_state.iter() {
@@ -7231,9 +7232,12 @@ where
 				for (chan_id, chan) in peer_state.channel_by_id.iter_mut()
 					.filter_map(|(chan_id, chan)| chan.as_funded_mut().map(|chan| (chan_id, chan)))
 				{
-					let new_feerate = selected_commitment_sat_per_1000_weight(
-						&self.fee_estimator, chan.funding.get_channel_type(),
-					);
+					let channel_type = chan.funding.get_channel_type();
+					let new_feerate = feerate_cache.get(channel_type).copied().or_else(|| {
+						let feerate = selected_commitment_sat_per_1000_weight(&self.fee_estimator, &channel_type);
+						feerate_cache.insert(channel_type.clone(), feerate);
+						Some(feerate)
+					}).unwrap();
 					let chan_needs_persist = self.update_channel_fee(chan_id, chan, new_feerate);
 					if chan_needs_persist == NotifyOption::DoPersist { should_persist = NotifyOption::DoPersist; }
 				}
@@ -7271,6 +7275,7 @@ where
 			let mut handle_errors: Vec<(Result<(), _>, _)> = Vec::new();
 			let mut timed_out_mpp_htlcs = Vec::new();
 			let mut pending_peers_awaiting_removal = Vec::new();
+			let mut feerate_cache = new_hash_map();
 
 			{
 				let per_peer_state = self.per_peer_state.read().unwrap();
@@ -7282,9 +7287,12 @@ where
 					peer_state.channel_by_id.retain(|chan_id, chan| {
 						match chan.as_funded_mut() {
 							Some(funded_chan) => {
-								let new_feerate = selected_commitment_sat_per_1000_weight(
-									&self.fee_estimator, funded_chan.funding.get_channel_type(),
-								);
+								let channel_type = funded_chan.funding.get_channel_type();
+								let new_feerate = feerate_cache.get(channel_type).copied().or_else(|| {
+									let feerate = selected_commitment_sat_per_1000_weight(&self.fee_estimator, &channel_type);
+									feerate_cache.insert(channel_type.clone(), feerate);
+									Some(feerate)
+								}).unwrap();
 								let chan_needs_persist = self.update_channel_fee(chan_id, funded_chan, new_feerate);
 								if chan_needs_persist == NotifyOption::DoPersist { should_persist = NotifyOption::DoPersist; }