#625 In order to fix the deadlock, reviewed the `[SDImageCache diskImageExistsWithKey:]` method. Based on the Apple doc for NSFileManager, using the defaultManager without the dispatch on the ioQueue to avoid the deadlocks. This instance is thread safe. Also created an async variant of this method `[SDImageCache diskImageExistsWithKey:completion:]`
For consistency, added async methods in `SDWebImageManager` `cachedImageExistsForURL:completion:` and `diskImageExistsForURL:completion:`
Replace #699 Fixed race condition in SDWebImageManager if one operation is cancelled, the completion block must not be called, otherwise it might race with a newer completion for the same object

Conflicts:
	SDWebImage/SDWebImageManager.m