Avoid spinning when workers have no dataflows

[frankmcsherry]

Meant to address the same issue as #461, and up for discussion.

The intent here is that calling worker.step_or_park(duration) should not ignore the argument when worker.dataflows is empty. It chose to, because it our shutdown code we end up in a loop that won't break out there are no dataflows (and nothing to wake us). Some head scratching later, I deemed this to be a bug in the shutdown code: we shouldn't enter the loop if there are no dataflows, and if there are dataflows then we will exit it (as reaching zero dataflows is the test to break out of the loop).

Said differently, the code should probably have been:

while worker.has_dataflows() {
    worker.step_or_park(None);
}

and .. we could still do that. This PR does a similar change that is not quite as clear exactly that.