refactor the way tasks are scheduled in event-loop

[debris]

after #8185

Current design requires us to define all tasks types in a single place in ClientIoMessage. It makes it difficult to modularize ethcore . Let's think how we could refactor that so tasks could be scheduled without the need of defining them in a single place.

cc @tomaka @0x7CFE

[tomaka]

Ideally the long-term plan would be to remove the current ethcore_io crate and use futures and streams.
If that happens, each individual message could have its own stream.

[0x7CFE]

@debris Recently I was skimming through a blog post describing Actix API. Maybe we may use something similar.

The key idea is that instead of huge enums messages and handlers may be defined as trait impls:

use actix::prelude::*;

// `PlusOne` message implementation

struct PlusOne;

impl Message for PlusOne {
    type Result = u32;
}

// `CounterActor` implementation

struct CounterActor {
    count: u32,
}

impl CounterActor {
    pub fn new() -> CounterActor {
        CounterActor { count: 0 }
    }
}

impl Actor for CounterActor {
    type Context = Context<Self>;
}

impl Handler<PlusOne> for CounterActor {
    type Result = u32;

    fn handle(&mut self, _msg: PlusOne, _ctx: &mut Context<Self>) -> u32 {
        self.count += 1;
        self.count
    }
}

That way code would be organized more naturally.

[adria0]

Closing the issue due to its stale state