Consider providing a hook into communication thread spawning

For example, this would enable pinning communication threads.

This could be done by allowing customisation of how comm threads are spawned by modifying a Configuration::Cluster (similarly to how comm logging is set-up).

Box::new(|index, sender, _, loop_fn| {
    ::std::thread::Builder::new()
        .name(
            if sender { format!("send thread {}", index) }
            else { format!("send thread {}", index) })
        .spawn(move || {
          pin_to_core(base + index);
          loop_fn.start()
        })
}),

Other designs are possible/desirable.

https://github.com/TimelyDataflow/timely-dataflow/pull/203 has an early attempt.

Start by reviewing Configuration::Cluster and the existing communication-thread logging setup, then compare the early attempt in pull request #203. Determine a suitable hook for customizing communication-thread spawning, including the callback inputs and thread startup behavior. Done means the design is integrated without breaking existing spawning behavior and supports the proposed thread pinning use case.