fix NSInternalInconsistencyException crash

[whrlgus]

I experienced an issue "Invalid batch updates detected", when the main thread was busy working.

The photoLibraryDidChange function would be called multiple times on background thread and changes fetched by old fetchResult caused NSInternalInconsistencyException crash in async main dispatch block.

This change eliminates the possibility that fetched changes will be stale. 😀

[whrlgus]

reproduce crash:

1. use sleep function in async main dispatch block to make the process take long for completion
2. build your app and present a photo picker view
3. move to album app, delete few photos in your album and take a photo using a camera app
4. return to your app, then your app will crash