Issue/2596 fixes coredata threading glitch

Fixes #2596

This assertion will be valid only in debug calls, and would help us spot any bugs. didChangeAccount notification is posted [here](https://github.com/wordpress-mobile/WordPress-iOS/blob/release/4.5/WordPress/Classes/Services/AccountService.m#L94) and [here](https://github.com/wordpress-mobile/WordPress-iOS/blob/release/4.5/WordPress/Classes/Services/AccountService.m#L111).

As for the `readerTopicDidChange`, it gets posted in the `ReaderTopicService` class, from the main thread. I believe adding a dispatch_async call here would be redundant (imho!)