Provide our peers with SCID aliases and forward payments with them

Provide our peers with SCID aliases and forward payments with them

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.