Debug-log monitor update completions in ChainMonitor

When a `ChannelMonitor[Update]` persistence completes, we rely on
logging in `ChannelManager` to hear about it. However, this won't
happen at all if there's still pending updates as no `MonitorEvent`
will be generated.

Thus, here, we add logging directly in `ChainMonitor`, ensuring we
can deduce when individual updates completed from debug logs.

Author: TheBlueMatt

lightning/src/chain/chainmonitor.rs

@@ -537,6 +537,13 @@ where C::Target: chain::Filter,
 				// `MonitorEvent`s from the monitor back to the `ChannelManager` until they
 				// complete.
 				let monitor_is_pending_updates = monitor_data.has_pending_offchain_updates(&pending_monitor_updates);
+				log_debug!(self.logger, "Completed off-chain monitor update for channel with funding outpoint {:?}, {}",
+					funding_txo,
+					if monitor_is_pending_updates {
+						"still have pending off-chain updates"
+					} else {
+						"all off-chain updates complete, returning a MonitorEvent"
+					});
 				if monitor_is_pending_updates {
 					// If there are still monitor updates pending, we cannot yet construct a
 					// Completed event.
@@ -548,7 +555,16 @@ where C::Target: chain::Filter,
 				}], monitor_data.monitor.get_counterparty_node_id()));
 			},
 			MonitorUpdateId { contents: UpdateOrigin::ChainSync(_) } => {
-				if !monitor_data.has_pending_chainsync_updates(&pending_monitor_updates) {
+				let monitor_has_pending_updates =
+					monitor_data.has_pending_chainsync_updates(&pending_monitor_updates);
+				log_debug!(self.logger, "Completed chain sync monitor update for channel with funding outpoint {:?}, {}",
+					funding_txo,
+					if monitor_has_pending_updates {
+						"still have pending chain sync updates"
+					} else {
+						"all chain sync updates complete, releasing pending MonitorEvents"
+					});
+				if !monitor_has_pending_updates {
 					monitor_data.last_chain_persist_height.store(self.highest_chain_height.load(Ordering::Acquire), Ordering::Release);
 					// The next time release_pending_monitor_events is called, any events for this
 					// ChannelMonitor will be returned.