Prevent ChannelForceClosed monitor update error after detecting spend

If we detected a spend for a channel onchain prior to handling its
`ChannelForceClosed` monitor update, we'd log a concerning error
message and return an error unnecessarily. The channel has already been
closed, so handling the `ChannelForceClosed` monitor update at this
point should be a no-op.

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -2457,7 +2457,9 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 
 		self.latest_update_id = updates.update_id;
 
-		if ret.is_ok() && self.funding_spend_seen {
+		// Refuse updates after we've detected a spend onchain, but only if we haven't processed a
+		// force closed monitor update yet.
+		if ret.is_ok() && self.funding_spend_seen && self.latest_update_id != CLOSED_CHANNEL_UPDATE_ID {
 			log_error!(logger, "Refusing Channel Monitor Update as counterparty attempted to update commitment after funding was spent");
 			Err(())
 		} else { ret }