Deduplicating Processing Tasks

[kean]

• Add isProcessingDeduplicationEnabled option:

    `false` by default. An experimental feature. When enabled, pipeline
    will avoid any duplicated work when processing images. For example,
    let't take this two requests:
    
    ```swift
    let url = URL(string: "http://example.com/image")
    pipeline.loadImage(with: ImageRequest(url: url, processors: [
        ImageProcessor.Resize(size: CGSize(width: 44, height: 44)),
        ImageProcessor.GaussianBlur(radius: 8)
    ]))
    pipeline.loadImage(with: ImageRequest(url: url, processors: [
        ImageProcessor.Resize(size: CGSize(width: 44, height: 44))
    ]))
    ```
    
    Nuke will load the image data only once, resize the image once and
    apply the blur also only once. There is no duplicated work done at
    any stage.
    
    - WARNING: When enabled each intermediate processed image will be
    stored in disk data cache if it is enabled.