don't require Sync trait for job function

[mike1703]

What is the reason for the Sync trait for the function a job executes?
I understand the need for Send, but I don't exactly understand why I need Sync here.

Sync would be necessary if I want to share data between e.g. threads. But this is not really necessary from my perspective.
I want to be able to use std::sync::mpsc to regularly send some commands in a Sender.
Sender implements Send but no Sync. To use a Sender from within the function called from the job I would need to move the necessary variables inside the closure.

example code:

let command_tx: Sender<String> = ...;
scheduler.add(Job::new("1/10 * * * * *".parse().unwrap(), {
    move || {
        command_tx
            .send("I get executed every 10 seconds!".into())
            .unwrap();
    }
}));

without my commit this produces the following error:

`std::sync::mpsc::Sender<std::string::String>` cannot be shared between threads safely

I might overlook some important part here, but for my example code this worked out fine.

[BlackDex]

Thanks @mike1703. You are right, Sync is not needed at all.
I also have created an example using MPSC. That should help for others and future updates to not have any issues with your use case.

[mike1703]

Thank you for merging! :)