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

@@ -8441,7 +8441,7 @@ where
 								funding_txo, monitor, peer_state, ""));
 					}
 				}
-				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",