Issue/2596 fixes coredata threading glitch

Fixes #2596

I see `updateAndPerformFetchRequest` is being called from a couple `NSNotification` observers, which might be called in a background thread (`readerTopicDidChange:` and `didChangeAccount:`).

They probably aren't right now, but they could. They also call `self.tableView` so there's good potential for crashing :see_no_evil: