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 
Hitting this assert.
```
	0   CoreFoundation                      0x0711d1e4 __exceptionPreprocess + 180
	1   libobjc.A.dylib                     0x06de68e5 objc_exception_throw + 44
	2   CoreFoundation                      0x0711d048 +[NSException raise:format:arguments:] + 136
	3   Foundation                          0x050084de -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 116
	4   Paper Dev                           0x01068614 -[AFJSONRequestOperation error] + 228
	5   Paper Dev                           0x0106fc05 -[AFURLConnectionOperation cancelConnection] + 629
	6   libobjc.A.dylib                     0x06df882b -[NSObject performSelector:withObject:] + 70
	7   Foundation                          0x0507ce48 __NSThreadPerformPerform + 285
	8   CoreFoundation                      0x070a677f __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
	9   CoreFoundation                      0x070a610b __CFRunLoopDoSources0 + 235
	10  CoreFoundation                      0x070c31ae __CFRunLoopRun + 910
	11  CoreFoundation                      0x070c29d3 CFRunLoopRunSpecific + 467
	12  CoreFoundation                      0x070c27eb CFRunLoopRunInMode + 123
	13  Foundation                          0x05084e35 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 284
	14  Foundation                          0x05020dc1 -[NSRunLoop(NSRunLoop) run] + 82
	15  Paper Dev                           0x0106c23b +[AFURLConnectionOperation networkRequestThreadEntryPoint:] + 123
	16  Foundation                          0x0507fa07 -[NSThread main] + 76
	17  Foundation                          0x0507f966 __NSThread__main__ + 1275
	18  libsystem_pthread.dylib             0x0775f5fb _pthread_body + 144
	19  libsystem_pthread.dylib             0x0775f485 _pthread_struct_init + 0
	20  libsystem_pthread.dylib             0x07764cf2 thread_start + 34
)

```