Avoid refusing ChannelMonitorUpdates we expect to receive after closing

There is no need to fill the user's logs with errors that are expected
to be hit based on specific edge cases, like providing preimages after
a monitor has seen a confirmed commitment on-chain.

This doesn't really change our behavior – we still apply and persist the
state changes resulting from processing these updates regardless of
whether they succeed or not.

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -2370,6 +2370,13 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 				},
 			}
 		}
+
+		// If the updates succeeded and we were in an already closed channel state, then there's no
+		// need to refuse any updates we expect to receive afer seeing a confirmed commitment.
+		if ret.is_ok() && updates.update_id == CLOSED_CHANNEL_UPDATE_ID && self.latest_update_id == updates.update_id {
+			return Ok(());
+		}
+
 		self.latest_update_id = updates.update_id;
 
 		if ret.is_ok() && self.funding_spend_seen {