XXX: Track SCID aliases from our counterparty and use them in invoices

XXX: 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.

XXX: Note in spec pr that we need to drop the "MUST NOT retransmit funding_locked"
message in reconnection if we are passed initial reconnect. This may
imply a feature bit, but will at least imply some kind of "MUST NOT
retransmit if it doesn't have evidence the counterparty interpreted the
SCID alias previously sent" (to avoid backwards incompat).