debugStats = pool => {
  const {totalWorkers, busyWorkers, idleWorkers, pendingTasks} = pool.stats();
  debug(
    '%d/%d busy workers; %d idle; %d tasks queued',
    busyWorkers,
    totalWorkers,
    idleWorkers,
    pendingTasks
  );
}
Outputs a debug statement with worker stats
@param {BufferedWorkerPool} pool - Worker pool