Refactor tracker modes: decouple privacy from whitelist features

[josecelano]

These are the current 4 tracker modes:

pub enum TrackerMode {
    /// Will track every new info hash and serve every peer.
    #[serde(rename = "public")]
    Public,

    /// Will only track whitelisted info hashes.
    #[serde(rename = "listed")]
    Listed,

    /// Will only serve authenticated peers
    #[serde(rename = "private")]
    Private,

    /// Will only track whitelisted info hashes and serve authenticated peers
    #[serde(rename = "private_listed")]
    PrivateListed,
}

I think we have discussed this before. I think we could split the mode into two different structs:

pub enum TrackerPrivacy {
    /// Will track every new info hash and serve every peer.
    #[serde(rename = "public")]
    Public,

    /// Will only serve authenticated peers
    #[serde(rename = "private")]
    Private,
}

pub struct Configuration {
    // ...

    pub mode: TrackerPrivacy,
    pub is_whitelisted: bool,

    // ...
}

mode = "private"
is_whitelisted = true

There is a lot of ambiguity in the code because sometimes we refer to "public" tracker not only for the TrackerMode::public but also for the TrackerMode::Listed.

I have started using these words to reduce the ambiguity:

• Open: public and listed
• Close: private and private listed.

But in the end, I think those are totally independent features.

cc @torrust/torrustaceans