Revoke SSH/GPG keys asynchronously

When an SSH/GPG key is revoked, we need to update its signatures as unverified:

• SSH keys
• GPG keys

Usually, SSH/GPG keys don't have a huge number of signatures and the update is fast enough to perform sync. But if a key has a large number signatures it may become a problem: !108344 (comment 1241627100).

Let's move the calculation into a worker.