`confidential_key` is a misleading name

I think `confidential_key` should be renamed because it refers to an object which is not a confidential key: it's a key but it's not confidential.
Suggestions:
* `scanning_pubkey` because the corresponding private key can be used for scanning (is this actually true?)
* `encryption_pubkey` or `ecdh_pubkey` because the pubkey is used to encrypt data for the receiver