Ack all monitor events besides HTLCUpdate

Currently, the resolution of HTLCs (and decisions on when HTLCs can be
forwarded) is the responsibility of Channel objects (a part of ChannelManager)
until the channel is closed, and then the ChannelMonitor thereafter. This leads
to some complexity around race conditions for HTLCs right around channel
closure. Additionally, there is lots of complexity reconstructing the state of
all HTLCs in the ChannelManager deserialization/loading logic.

Instead, we want to do all resolution in ChannelMonitors (in response to
ChannelMonitorUpdates) and pass them back to ChannelManager in the form of
MonitorEvents (similar to how HTLCs are resolved after channels are closed). In
order to have reliable resolution, we'll need to keep MonitorEvents around in
the ChannelMonitor until the ChannelManager has finished processing them. This
will simplify things - on restart instead of examining the set of HTLCs in
monitors we can simply replay all the pending MonitorEvents.

Here we take care of the monitor events that are trivial to ACK, since all
except HTLCEvents can be ACK'd immediately after the event is handled and don't
need to be re-processed on startup.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -13267,7 +13267,8 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 		for (funding_outpoint, channel_id, mut monitor_events, counterparty_node_id) in
 			pending_monitor_events.drain(..)
 		{
-			for (_event_id, monitor_event) in monitor_events.drain(..) {
+			for (event_id, monitor_event) in monitor_events.drain(..) {
+				let monitor_event_source = MonitorEventSource { event_id, channel_id };
 				match monitor_event {
 					MonitorEvent::HTLCEvent(htlc_update) => {
 						let logger = WithContext::from(
@@ -13350,6 +13351,9 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 								failed_channels.push((Err(e), counterparty_node_id));
 							}
 						}
+						// Channel close monitor events do not need to be replayed on startup because we
+						// already check the monitors to see if the channel is closed.
+						self.chain_monitor.ack_monitor_event(monitor_event_source);
 					},
 					MonitorEvent::CommitmentTxConfirmed(_) => {
 						let per_peer_state = self.per_peer_state.read().unwrap();
@@ -13371,13 +13375,17 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 								failed_channels.push((Err(e), counterparty_node_id));
 							}
 						}
+						// Channel close monitor events do not need to be replayed on startup because we
+						// already check the monitors to see if the channel is closed.
+						self.chain_monitor.ack_monitor_event(monitor_event_source);
 					},
 					MonitorEvent::Completed { channel_id, monitor_update_id, .. } => {
 						self.channel_monitor_updated(
 							&channel_id,
 							Some(monitor_update_id),
 							&counterparty_node_id,
 						);
+						self.chain_monitor.ack_monitor_event(monitor_event_source);
 					},
 				}
 			}