Replace NullStore with InMemoryFeedStore to improve UX

- The InMemoryStore can provide better UX as it supports pagination and image caching
- Made the infra queue serial - instead of concurrent - since the InMemoryFeedStore is not thread safe

EssentialApp/EssentialApp/SceneDelegate.swift

@@ -18,8 +18,7 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
 
 		return DispatchQueue(
 			label: "com.essentialdeveloper.infra.queue",
-			qos: .userInitiated,
-			attributes: .concurrent
+			qos: .userInitiated
 		).eraseToAnyScheduler()
 	}()
 
@@ -38,7 +37,7 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
 		} catch {
 			assertionFailure("Failed to instantiate CoreData store with error: \(error.localizedDescription)")
 			logger.fault("Failed to instantiate CoreData store with error: \(error.localizedDescription)")
-			return NullStore()
+			return InMemoryFeedStore()
 		}
 	}()