Cleanup AVCaptureSession on ZXCapture dealloc

I notices significant delays to scanning and AVCaptureVideoPreviewLayer display after instantiating a ZXCapture, scanning, dealloc, and then instantiating a second ZXCapture object.  Cleaning up AVCaptureSession inputs and outputs before ZXCapture deallocation removed any delays and the occasional inability to display the AVCaptureVideoPreviewLayer correctly.