Rename Channel's latest-monitor-update fetch method for clarity

`Channel::get_latest_complete_monitor_update_id` no longer refers
to complete updates, but rather ones which were passed to the
`ChannelManager` and which the `CHannel` no longer knows about.
Thus, we rename it `get_latest_unblocked_monitor_update_id`.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -4404,7 +4404,8 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		(self.context.channel_state & ChannelState::MonitorUpdateInProgress as u32) != 0
 	}
 
-	pub fn get_latest_complete_monitor_update_id(&self) -> u64 {
+	/// Gets the latest [`ChannelMonitorUpdate`] ID which has been released and is in-flight.
+	pub fn get_latest_unblocked_monitor_update_id(&self) -> u64 {
 		if self.context.pending_monitor_updates.is_empty() { return self.context.get_latest_monitor_update_id(); }
 		self.context.pending_monitor_updates[0].update.update_id - 1
 	}

lightning/src/ln/channelmanager.rs

@@ -8421,12 +8421,12 @@ where
 							peer_state.in_flight_monitor_updates.insert(funding_txo, chan_in_flight_upds);
 						}
 					}
-					if chan.get_latest_complete_monitor_update_id() > max_in_flight_update_id {
+					if chan.get_latest_unblocked_monitor_update_id() > max_in_flight_update_id {
 						// If the channel is ahead of the monitor, return InvalidValue:
 						log_error!(args.logger, "A ChannelMonitor is stale compared to the current ChannelManager! This indicates a potentially-critical violation of the chain::Watch API!");
 						log_error!(args.logger, " The ChannelMonitor for channel {} is at update_id {} with update_id through {} in-flight",
 							log_bytes!(chan.context.channel_id()), monitor.get_latest_update_id(), max_in_flight_update_id);
-						log_error!(args.logger, " but the ChannelManager is at update_id {}.", chan.get_latest_complete_monitor_update_id());
+						log_error!(args.logger, " but the ChannelManager is at update_id {}.", chan.get_latest_unblocked_monitor_update_id());
 						log_error!(args.logger, " The chain::Watch API *requires* that monitors are persisted durably before returning,");
 						log_error!(args.logger, " client applications must ensure that ChannelMonitor data is always available and the latest to avoid funds loss!");
 						log_error!(args.logger, " Without the latest ChannelMonitor we cannot continue without risking funds.");