SCIDs in forward_htlcs (and decode_update_add_htlcs) can change out from under us #4121
Description
If we forward an HTLC from a channel and then splice that channel before the HTLC is resolved, we'll end up with an HTLCSource::Forward with a short_channel_id that doesn't correspond to any channel now open. AFAICT this can result in the HTLC resolution getting lost. It seems like the simple solution is to replace the SCIDs with the outbound SCID aliases which are static per-channel, but it would be nice to change the maps to use (PublicKey, ChannelId) like everything else now.