code: function callNext() {
  			/* istanbul ignore next */
  			if (queue.length == 0) return;
  
  			/* istanbul ignore next */
  			if (currentInFlight >= opts.concurrency) return;
  
  			const item = queue.shift();
  
  			currentInFlight++;
  			handler(item.ctx)
  				.then(res => {
  					currentInFlight--;
  					item.resolve(res);
  					callNext();
  				})
  				.catch(err => {
  					currentInFlight--;
  					item.reject(err);
  					callNext();
  				});
  		}
docs: Call the next request from the queue