title: captureStillImageAsynchronously at specific time not working
body: <p>I am writing an application that takes an images every 0.1 seconds after the user presses a button.  However I'm having trouble getting a consistent 0.1 second delay between images.  </p>
  
  <p>Is this due to the fact that <code>captureStillImageAsynchronously</code> is asynchronous closure?  Would switching to <code>capturePhoto</code> from <code>AVCapturePhotoOutput</code> fix this issue?  Here is a summary of my code.</p>
  
  <pre><code>var stillImageOutput : AVCaptureStillImageOutput?
  
  // Button action.
   @IBAction func takeImage(_ sender: UIButton) {      
      //take 1 image every 0.1 seconds scheduled using a timer
      Timer.scheduledTimer(timeInterval: 0.1,
                          target: self,
                          selector: #selector(calldidPressTakePhoto),
                          userInfo: nil,
                          repeats: true)
  }
  
  // Take image
      func didPressTakePhoto(){
  
          if let videoConnection = stillImageOutput?.connection(withMediaType: AVMediaTypeVideo){
  
              videoConnection.videoOrientation = AVCaptureVideoOrientation.portrait
  
              //Capture image.  everything in this closure is ASYNCHRONOUS?
              stillImageOutput?.captureStillImageAsynchronously(from: videoConnection, completionHandler: {
                  (sampleBuffer, error) in
  
                  //Do something with the result image...
  
              })
          }
      }
  </code></pre>