title: FetchedResultsController blocks UI when its NSManagedObjectContext merging with another one in background thread
body: <p>I have a tableViewController that conforms to NSFetchedResultsController protocol. 
  After I downloading data from my server using a background thread to save in to Core Data I have to merge changes to my main thread NSManagedObject context used by an instance of NSFetchedResultsController in this tableView. Right after merging the UI is blocked(For a few seconds).(NSFetchedResultsController is updating). 
  I am wondering what has caused this blocking?
  Is there any way to bypass this blocking?</p>
  
  <p>Because I think this NSFetchedResultsController updating is gonna update the UI so I assume It's not appropriate to place it in another thread.</p>
  
  <p>Thank you guys.</p>