Fix incorrect concurrency when handling NSManagedObjectDidSaveNotification's

Apple's docs for NSManagedObjectContextDidSaveNotification [state](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObjectContext_Class/NSManagedObjectContext.html#//apple_ref/c/data/NSManagedObjectContextDidSaveNotification):

	"You can only use the managed objects in this notification on the same thread on which it was posted."

Therefore, -performBlock should not be used to handle these notifications, but performBlockAndWait.

This change removes all errors detected by [GDCoreDataConcurrencyDebugging](https://github.com/GrahamDennis/GDCoreDataConcurrencyDebugging)