add Worker trait

[pvdrz]

fixes #234.

[pvdrz]

@thibault-martinez maybe we could try the for_each_concurrent here?

[zesterer]

I wonder whether we could separate run into distinct start and stop methods such that we can know externally when each worker has started and when it has finished its shutdown (right now it's a bit opaque and this is going to be annoying to work with when we want more granular control over startup/shutdown)

[pvdrz]

@zesterer what should stop do? I think most (all?) workers just break the loop and write the Stopped log entry.

[zesterer]

Yep, stop would break the loop. I do wonder whether the existing shutdown system might have a part to play here instead though.

[zesterer]

I don't think that Event/Receiver should exist here. Doing things through the event bus by passing a reference to the node in might work better here. Something like this:

impl<N: Node> Worker<N> for MyWorker {
    fn start(node: &N) -> Result<Self, Self::Error> {
        let (tx, rx) = channel();
        node.event_bus().register_handler::<MyEvent>(move |e| tx.send(e.clone()));

        task::spawn(move || {
            while let Ok(e) = rx.recv().await {
                // Use `e` here
            }
        });

        Ok(Self)
    }
}

Additionally, since this is a pretty common use-case, a utility method on EventBus like EventBus::register_receiver<E>() -> Receiver<E> would be quite a neat way to simplify this case to the following:

impl<N: Node> Worker<N> for MyWorker {
    fn start(node: &N) -> Result<Self, Self::Error> {
        let rx = node.event_bus().register_receiver::<MyEvent>();

        task::spawn(move || {
            while let Ok(e) = rx.recv().await {
                // Use `e` here
            }
        });

        Ok(Self)
    }
}

[zesterer]

I think I mentioned this before, but I'd really like to replace all of this sort of thing with a central event bus + a way to get references to other workers via the N: Node. Probably for a follow-up PR though.