Fix SDWebImageDownloadOperation imageData multi-thread issue.

[dreampiggy]

Force acess to imageData on the same delegate queue to solve non thread safe issue. And ensure image process and completion block with non-mutable data

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This merge request fixes / reffers to the following issues: ...

Pull Request Description

This PR is about #1998

The NSMutableData imageData can be set to nil during -(void)reset call and some call in URLSessionDelegate. So there may be an data race that the reset queue and delegate queue is not equal, and this can cause multi-thread problem.

We can make all the access(getter and setter) to imageData into an single queue to ensure this. But since all the acess to imageData is under NSURLSessionDelegate call, so we just need to change that self.imageData = nil in to the same delegate queue. So I get the queue from the session and put that nil process into it.

And moreover. The imageData is mutable but the completion block is an immutable NSData. We should always use copy to get the immutable instance to avoid modification while process.(Since NSData supports Copy on Write and many cases this would not cause any performance issue)

[bpoplauschi]

Looks good @dreampiggy, thanks.