Fix issue with AFMMRecordResponseSerializer threading

I created a new pull request with some additional error handling using the built in debugger. @jonbrooks please take a look.
Fascinating.  +100 for teaching me about CoreData via this issue/PR :)

IMO, the MMRecord.m approach of observing merge events is cleaner and more "correct".  It seems like this kind of thing needs to be done right/carefully or people are going to run into weird & unexpected bugs.

Looking at the implementation here:  https://github.com/mutualmobile/MMRecord/blob/master/Source/MMRecord/MMRecord.m#L1003
... it really doesn't seem too bad.  (I might be missing some other piece)

Agree that it would be cool to move this up to some shared place, but even replicating this logic in AFMMRecordResponseSerializer seems reasonable to me.