When we fail an HTLC which was destined for a channel that the HTLC
sender didn't know the real SCID for, we should ensure we continue
to use the alias in the channel_update we provide them. Otherwise
we will leak the channel's real SCID to HTLC senders.
As a part of adding SCID aliases to channels, we now have to accept
otherwise-redundant funding_locked messages which serve only to
update the SCID alias. Previously, we'd failt he channel as such
an update used to be bogus.
This creates an SCID alias for all of our outbound channels, which
we send to our counterparties as a part of the `funding_locked`
message and then recognize in any HTLC forwarding instructions.
Note that we generate an SCID alias for all channels, including
already open ones, even though we currently have no way of
communicating to our peers the SCID alias for already-open
channels.
