add a convenience function to make named dataflows with otherwise default arguments

[elindsey]

I'm naming the logging dataflows in materialize and keep writing boilerplate like

    let worker_logging = worker.log_register().get("timely");
    let traces = worker.dataflow_core(
        "Dataflow: differential logging",
        worker_logging,
        Box::new(()),
        move |_, scope| {});

Which could instead be

let traces = worker.dataflow_named("Dataflow: differential logging", move |scope| {});

[frankmcsherry]

Seems like a great idea, thanks!

[elindsey]

When convenient, could I get a merge? 🙂