Function should_keep_new_backup 

fn should_keep_new_backup(
    current_backup: Option<&KeyBackupData>,
    new_backup: &KeyBackupData,
) -> bool

Checks whether we should replace an existing backup’s data with the given new one.

First, if there is no existing current backup, we of course want to keep the new one.

Then, the rules to decide whether to keep the existing one or to replace it are:

• If the keys have different values for is_verified, keep the one which is_verified = true
• Otherwise if they are identical, keep the key with the lowest first_message_index
• Otherwise if they are identical, keep the key with the lowest forwarded_count