A database field was already in place to allow channel selection for
loop in. Unfortunately this field, which contains a short channel id,
isn't easily usable for controlling the loop server payment. Because of
non-strict forwarding, it is only possible to constrain a route to a
specific last hop pubkey.
This commit converts the existing field into a pubkey field.
We update to new set of server protos where new Terms calls are added.
Here static information will be returned from the server.
We no longer have a feebase+feerate, but get a final fee directly
returned by the server.
Allow user to specify that htlc will be published by an external source.
In that case no internal htlc tx will be published.
To facilitate sending to the htlc address, the swap initiation response
is extended with that address.
