Attempt to unblock blocked monitor updates on startup

When we make an MPP claim we push RAA blockers for each chanel to
ensure we don't allow any single channel to make too much progress
until all channels have the preimage durably on disk. We don't have
to store those RAA blockers on disk in the ChannelManager as
there's no point - if the ChannelManager gets to disk with the RAA
blockers it also brought with it the pending ChannelMonitorUpdates
that contain the preimages and will now be replayed, ensuring the
preimage makes it to all ChannelMonitors.

However, just because those RAA blockers dissapear on reload
doesn't mean the implications of them does too - if a later
ChannelMonitorUpdate was blocked in the channel we don't have logic
to unblock it on startup.

Here we add such logic, simply attempting to unblock all blocked
`ChannelMonitorUpdate`s that existed on startup.

Code written by Claude.

Fixes #4518

Author: Matt Corallo

lightning/src/ln/channelmanager.rs

@@ -1473,6 +1473,11 @@ enum BackgroundEvent {
 		channel_id: ChannelId,
 		highest_update_id_completed: u64,
 	},
+	/// A channel had blocked monitor updates waiting on startup. If the updates were blocked on
+	/// an MPP claim blocker not written to disk, we may be able to unblock them now.
+	///
+	/// This event is never written to disk.
+	AttemptUnblockMonitorUpdates { counterparty_node_id: PublicKey, channel_id: ChannelId },
 }
 
 /// A pointer to a channel that is unblocked when an event is surfaced
@@ -8795,6 +8800,12 @@ impl<
 						&counterparty_node_id,
 					);
 				},
+				BackgroundEvent::AttemptUnblockMonitorUpdates {
+					counterparty_node_id,
+					channel_id,
+				} => {
+					self.handle_monitor_update_release(counterparty_node_id, channel_id, None);
+				},
 			}
 		}
 		NotifyOption::DoPersist
@@ -9751,6 +9762,7 @@ impl<
 									BackgroundEvent::MonitorUpdatesComplete {
 										channel_id, ..
 									} => *channel_id == _prev_channel_id,
+									BackgroundEvent::AttemptUnblockMonitorUpdates { .. } => false,
 								}
 							});
 						assert!(channel_closed || matching_bg_event, "{:?}", *background_events);
@@ -19456,6 +19468,14 @@ impl<
 						log_error!(logger, " Please ensure the chain::Watch API requirements are met and file a bug report at https://github.com/lightningdevkit/rust-lightning");
 						return Err(DecodeError::DangerousValue);
 					}
+					if funded_chan.blocked_monitor_updates_pending() > 0 {
+						pending_background_events.push(
+							BackgroundEvent::AttemptUnblockMonitorUpdates {
+								counterparty_node_id: *counterparty_id,
+								channel_id: *chan_id,
+							},
+						);
+					}
 				} else {
 					// We shouldn't have persisted (or read) any unfunded channel types so none should have been
 					// created in this `channel_by_id` map.