[feature] PGP key transitions

[andres-erbsen]

It is a good idea to regenerate public keys every once in a while. Previously, the cumbersome nature of PGP web of trust certification has kept people from doing this as frequently as possible. With a certifying keyserver, this should no longer be an issue. Therefore, it would make sense to (eventually) have a specialized facility for performing a transition from one key to another. This would involve.

1. Adding a a new PGP public key to the keyserver profile
2. Setting the previous one to expire soon.
3. Waiting for outstanding email (signed under the old key) to be delivered.
4. Removing the old public key from the profile
5. Possibly waiting some more until it is believed that all mail encrypted to the old key has been received.
6. Securely erasing the old key.