Async jobs

[ivan770]

Currently, you can access shared state only in request scope.

This causes a lot of troubles, when you, for example, need to save state to file based on timer, or do any actions with state without HTTP request.

This PR introduces Job and JobContext, that represent async job and job context with app state respectively.

API is pretty straightforward:

#[async_std::main]
async fn main() -> Result<(), std::io::Error> {
    let mut app = tide::new();

    app.at("/").get(|_| async move {
        Ok("I am executed on HTTP request")
    });

    app.spawn(|_| async move {
        println!("I am executed when you call 'listen'!");
    });

    app.listen("127.0.0.1:8080").await?;
    Ok(())
}

Example with app state

[Fishrock123]

I think this was solved by #675 - you can now access state again off of the server, and so you could make an external task which does this.

Please let me know if I missed something though!