Add a TODO for an important issue for making async mon updates safe

If we receive a monitor event from a forwarded-to channel which
contains a preimage for an HTLC, we have to propogate that preimage
back to the forwarded-from channel monitor. However, once we have
that update, we're running in a relatively unsafe state - we have
the preimage in memory, but if we were to crash the forwarded-to
channel monitor will not regenerate the update with the preimage
for us. If we haven't managed to write the monitor update to the
forwarded-from channel by that point, we've lost the preimage, and,
thus, money!

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4318,8 +4318,12 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					// event being update_fulfill_htlc).
 					let update_res = self.chain_monitor.update_channel(prev_outpoint, preimage_update);
 					if update_res != ChannelMonitorUpdateResult::UpdateComplete {
+						// TODO: This needs to be handled somehow - if we receive a monitor update
+						// with a preimage we *must* somehow manage to propagate it to the upstream
+						// channel, or we must have an ability to receive the same event and try
+						// again on restart.
 						log_error!(self.logger, "Critical error: failed to update channel monitor with preimage {:?}: {:?}",
-											 payment_preimage, update_res);
+							payment_preimage, update_res);
 					}
 					// Note that we do *not* set `claimed_htlc` to false here. In fact, this
 					// totally could be a duplicate claim, but we have no way of knowing