Refactor ExpiringKey

[josecelano]

Relates to: #171

The ExpiringKey::valid_until (duration) does not need to be optional anymore since we introduced the KeyId struct.

Besides, after renaming the Key struct to ExpiringKey, there is no longer a conflict with the KeyId, so we can use the name Key for the KeyId again.

Before:

pub struct ExpiringKey {
    pub id: KeyId,
    pub valid_until: Option<DurationSinceUnixEpoch>,
}

After:

pub struct ExpiringKey {
    pub key: Key,
    pub valid_until: DurationSinceUnixEpoch,
}

After this refactoring, you can read the code like this:

"An ExpiringKey is a Key valid until a date expressed in a duration from the Unix epoch (timestamp)".

[josecelano]

Hi @da2ce7 @WarmBeer, this is ready to review. It's just a minor refactoring.

[da2ce7]

Looks good to me :)