Don't parent background managed object contexts to view context

Otherwise, certain operations require the background contexts to
interact with the view context, which can block the main thread from
accessing the view context (potentially causing hitches if the view
context access is in a critical path, like cell fetching).

Tusker/CoreData/MastodonCachePersistentStore.swift

@@ -20,13 +20,15 @@ class MastodonCachePersistentStore: NSPersistentContainer {
     
     private(set) lazy var backgroundContext: NSManagedObjectContext = {
         let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
-        context.parent = self.viewContext
+        context.persistentStoreCoordinator = self.persistentStoreCoordinator
+        context.automaticallyMergesChangesFromParent = true
         return context
     }()
     
     private(set) lazy var prefetchBackgroundContext: NSManagedObjectContext = {
         let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
-        context.parent = self.viewContext
+        context.persistentStoreCoordinator = self.persistentStoreCoordinator
+        context.automaticallyMergesChangesFromParent = true
         return context
     }()
     
@@ -51,6 +53,8 @@ class MastodonCachePersistentStore: NSPersistentContainer {
             }
         }
         
+        viewContext.automaticallyMergesChangesFromParent = true
+        
         NotificationCenter.default.addObserver(self, selector: #selector(managedObjectsDidChange), name: .NSManagedObjectContextObjectsDidChange, object: viewContext)
     }