main loop refactor: remove redundant queue and no active waiting for results

[banditopazzo]

This PR wants to make changes to improve the main loop.

Currently the main loop is using a VecDeque to schedule jobs on the threadpool, but the threadpool implements an internal queue itself, so basically there the code is redundant.

You can test this Threadpool behaviour with these simple lines:

fn main() {
    let pool = threadpool::ThreadPool::new(3);

    for x in 0..6 {
        pool.execute(move || {
            println!("start work {x}");

            std::thread::sleep(std::time::Duration::from_secs(3));

            println!("end work {x}");
        });
    }

    pool.join();
}

There is also a manual thread::sleep that can be avoided, relying on the Receiver::recv function and without actively checking for results in the channel receiver.

I think in general the main loop will be less convoluted.

[devttys0]

This is indeed much cleaner. :) Much obliged!