Add an additional variant to handle_new_monitor_update!

In d1c340a0e1 we added support in
`handle_new_monitor_update!` for handling updates without dropping
locks.

In the coming commits we'll start handling `ChannelMonitorUpdate`s
"like normal" for updates against closed channels. Here we set up
the first step by adding a new `POST_CHANNEL_CLOSE` variant on
`handle_new_monitor_update!` which attempts to handle the
`ChannelMonitorUpdate` and handles completion actions if it
finishes immediately, just like the pre-close variant.

lightning/src/ln/channelmanager.rs

@@ -3339,6 +3339,27 @@ macro_rules! handle_new_monitor_update {
 				let _ = in_flight_updates.remove(idx);
 			})
 	} };
+	(
+		$self: ident, $funding_txo: expr, $update: expr, $peer_state_lock: expr, $peer_state: expr,
+		$per_peer_state_lock: expr, $logger: expr, $channel_id: expr, POST_CHANNEL_CLOSE
+	) => { {
+		let in_flight_updates;
+		let idx;
+		handle_new_monitor_update!($self, $funding_txo, $update, $peer_state, $logger,
+			$channel_id, in_flight_updates, idx, _internal_outer,
+			{
+				let _ = in_flight_updates.remove(idx);
+				if in_flight_updates.is_empty() {
+					let update_actions = $peer_state.monitor_update_blocked_actions
+						.remove(&$channel_id).unwrap_or(Vec::new());
+
+					mem::drop($peer_state_lock);
+					mem::drop($per_peer_state_lock);
+
+					$self.handle_monitor_update_completion_actions(update_actions);
+				}
+			})
+	} };
 	(
 		$self: ident, $funding_txo: expr, $update: expr, $peer_state_lock: expr, $peer_state: expr,
 		$per_peer_state_lock: expr, $chan: expr