Handle cases where a channel is in use w/o an SCID in ChannelManager

In the next few commits we add support for 0conf channels, allowing
us to have an active channel with HTLC and other updates flying
prior to having an SCID available. This would break several
assumptions made in `ChannelManager`, which we address here by
looking at SCID aliases in addition to SCIDs.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1643,7 +1643,7 @@ macro_rules! handle_chan_restoration_locked {
 		let res = loop {
 			let forwards: Vec<(PendingHTLCInfo, u64)> = $pending_forwards; // Force type-checking to resolve
 			if !forwards.is_empty() {
-				htlc_forwards = Some(($channel_entry.get().get_short_channel_id().expect("We can't have pending forwards before funding confirmation"),
+				htlc_forwards = Some(($channel_entry.get().get_short_channel_id().unwrap_or($channel_entry.get().outbound_scid_alias()),
 					$channel_entry.get().get_funding_txo().unwrap(), forwards));
 			}
 
@@ -4908,7 +4908,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					break Ok((raa_updates.accepted_htlcs, raa_updates.failed_htlcs,
 							raa_updates.finalized_claimed_htlcs,
 							chan.get().get_short_channel_id()
-								.expect("RAA should only work on a short-id-available channel"),
+								.unwrap_or(chan.get().outbound_scid_alias()),
 							chan.get().get_funding_txo().unwrap()))
 				},
 				hash_map::Entry::Vacant(_) => break Err(MsgHandleErrInternal::send_err_msg_no_close("Failed to find corresponding channel".to_owned(), msg.channel_id))