Track SCID aliases from our counterparty and use them in invoices

Track SCID aliases from our counterparty and use them in invoices

New `funding_locked` messages can include SCID aliases which our
counterparty will recognize as "ours" for the purposes of relaying
transactions to us. This avoids telling the world about our
on-chain transactions every time we want to receive a payment, and
will allow for receiving payments before the funding transaction
appears on-chain.

Here we store the new SCID aliases and use them in invoices instead
of he "standard" SCIDs.