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 
> No one who cares about the final error state will expect "error" to return the final value before the operation finishes. 

Dunno, at least for me it seems like a logic error to check a value that you know will be nil. But for the sake of expediency, if you've inspected the usage and don't think it's troublesome, I'm OK with your proposal.