AFJSONRequestOperation processes JSON before isFinished set

The issue here was the `AFJSONRequestOperation`'s `responseJSON` method computed its result lazily. Therefore, if called from multiple threads, things would blow up. The solution proposed in this PR is to process the JSON prior to marking the operation finished, on the thread on which the connection finishes. This means that callers of `responseJSON` are not burdened with processing the JSON, and that property can be a simply read-only property.

Long-term, we're going to need to replace our use of AFNetworking 1.0. We may do this next milestone as we move towards using background transfer for backup/sync.

PTAL @floatplane @DawnWright @charlesmchen 
Check out [AFURLConnectionOperation cancelConnection].  It's what triggers the assert.  I feel like that's a legitimate usage of the error property.  We could easily rework that method to not use the error property... but I'm reluctant to change the semantics of error that much.  Especially since we don't have any known code in our codebase the erroneously accesses the error property too early.