Feature/6210 key newer

Use gnupg to merge keys if it's an update of an existing key. This is needed to prevent roll back attacks where the attacker push us to accept a key with an old expiration date that could be use to push an untrusted key when after it's expiration.
You can modify a key manually to remove parts of it, or use an older version of the key that didn't have some of the new stuff. For example you could have renewed the expiration date of a key, but an attacker might try to make us believe that the old key with the old expiration date is the newer version and then get us accept a key that is close to expire. After expiration we are way more relax on the rules of what we accept and the attacker could give as to accept an unrelated key that he controls.