Add support for NSPrivateQueueConcurrencyType

Currently, any context that uses the `NSPrivateQueueConcurrencyType` concurrency type deadlocks on `existingObjectWithID:error:` in [`insertOrUpdateObjects:ofEntity:withContext:error:completionBlock:`](https://github.com/sbonami/PFIncrementalStore/blob/master/PFIncrementalStore/PFIncrementalStore.m#L813). This is bad!