Macro seeders_loop

macro_rules! seeders_loop {
    ($seeders_sub:expr, $favored_seeder:expr, $code:expr) => { ... };
    ($seeders_sub:expr, $code:expr) => { ... };
}

Receive seeders from a subscription, and execute an async expression for each deduplicated seeder once (seeder order is random). It will keep going until the expression returns Ok(()), or there are no more seeders. It has an optional favored_seeder argument that will be tried first if specified.