[2.0] Refactor `completionQueue` / `completionBlock` to AFURLConnectionOperation

After adding `completionQueue` / `completionBlock` to `AFURLSessionManager`, I found myself wondering if it would be more consistent to move those properties to `AFURLConnectionOperation` from `AFHTTPRequestOperation`.

The one advantage to this approach is that vanilla `completionBlock`s can be run on a specified queue and group. It also appears to safeguard against potential threading issues in `nil`-ing out `completionBlock` to break the retain cycle.

Initial tests show this to work alright, but I'd love another set of eyes to make sure my understanding of GCD here is correct, and that this API is indeed desirable.