Add storage for ChannelMonitorUpdates in Channels

In order to support fully async `ChannelMonitor` updating, we need
to ensure that we can replay `ChannelMonitorUpdate`s if we shut
down after persisting a `ChannelManager` but without completing a
`ChannelMonitorUpdate` persistence. In order to support that we
(obviously) have to store the `ChannelMonitorUpdate`s in the
`ChannelManager`, which we do here inside the `Channel`.

We do so now because in the coming commits we will start using the
async persistence flow for all updates, and while we won't yet
support fully async monitor updating it's nice to get some of the
foundational structures in place now.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -743,6 +743,12 @@ pub(super) struct Channel<Signer: ChannelSigner> {
 	/// The unique identifier used to re-derive the private key material for the channel through
 	/// [`SignerProvider::derive_channel_signer`].
 	channel_keys_id: [u8; 32],
+
+	/// When we generate [`ChannelMonitorUpdate`]s to persist, they may not be persisted immediately.
+	/// If we then persist the [`channelmanager::ChannelManager`] and crash before the persistence
+	/// completes we still need to be able to complete the persistence. Thus, we have to keep a
+	/// copy of the [`ChannelMonitorUpdate`] here until it is complete.
+	pending_monitor_updates: Vec<ChannelMonitorUpdate>,
 }
 
 #[cfg(any(test, fuzzing))]
@@ -1112,6 +1118,8 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 
 			channel_type,
 			channel_keys_id,
+
+			pending_monitor_updates: Vec::new(),
 		})
 	}
 
@@ -1458,6 +1466,8 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 
 			channel_type,
 			channel_keys_id,
+
+			pending_monitor_updates: Vec::new(),
 		};
 
 		Ok(chan)
@@ -4891,6 +4901,10 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		(self.channel_state & ChannelState::MonitorUpdateInProgress as u32) != 0
 	}
 
+	pub fn get_next_monitor_update(&self) -> Option<&ChannelMonitorUpdate> {
+		self.pending_monitor_updates.first()
+	}
+
 	/// Returns true if funding_created was sent/received.
 	pub fn is_funding_initiated(&self) -> bool {
 		self.channel_state >= ChannelState::FundingSent as u32
@@ -6891,6 +6905,8 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 
 			channel_type: channel_type.unwrap(),
 			channel_keys_id,
+
+			pending_monitor_updates: Vec::new(),
 		})
 	}
 }