Opportunistically skip log in update_claims_view_from_matched_txn

On each block, for each `ChannelMonitor`, we log two status
statements in `OnChainTx::update_claims_view_from_matched_txn`.
This can add up to quite a bit, and is generally not very
interesting when we don't actually do anything if there's no claims
to bump.

Here we drop both logs if we have no claims to work with, but
retain it if we process any claims.

Author: TheBlueMatt

lightning/src/chain/onchaintx.rs

@@ -861,8 +861,15 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner> OnchainTxHandler<ChannelSigner>
 		B::Target: BroadcasterInterface,
 		F::Target: FeeEstimator,
 	{
-		log_debug!(logger, "Updating claims view at height {} with {} matched transactions in block {}", cur_height, txn_matched.len(), conf_height);
+		let mut have_logged_intro = false;
+		let mut maybe_log_intro = || {
+			if !have_logged_intro {
+				log_debug!(logger, "Updating claims view at height {} with {} matched transactions in block {}", cur_height, txn_matched.len(), conf_height);
+				have_logged_intro = true;
+			}
+		};
 		let mut bump_candidates = new_hash_map();
+		if !txn_matched.is_empty() { maybe_log_intro(); }
 		for tx in txn_matched {
 			// Scan all input to verify is one of the outpoint spent is of interest for us
 			let mut claimed_outputs_material = Vec::new();
@@ -955,6 +962,7 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner> OnchainTxHandler<ChannelSigner>
 			self.onchain_events_awaiting_threshold_conf.drain(..).collect::<Vec<_>>();
 		for entry in onchain_events_awaiting_threshold_conf {
 			if entry.has_reached_confirmation_threshold(cur_height) {
+				maybe_log_intro();
 				match entry.event {
 					OnchainEvent::Claim { claim_id } => {
 						// We may remove a whole set of claim outpoints here, as these one may have
@@ -992,7 +1000,11 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner> OnchainTxHandler<ChannelSigner>
 		}
 
 		// Build, bump and rebroadcast tx accordingly
-		log_trace!(logger, "Bumping {} candidates", bump_candidates.len());
+		if !bump_candidates.is_empty() {
+			maybe_log_intro();
+			log_trace!(logger, "Bumping {} candidates", bump_candidates.len());
+		}
+
 		for (claim_id, request) in bump_candidates.iter() {
 			if let Some((new_timer, new_feerate, bump_claim)) = self.generate_claim(
 				cur_height, &request, &FeerateStrategy::ForceBump, &*fee_estimator, &*logger,