Handle CKError.changeTokenExpired

[IngmarStein]

This PR handles CKError.changeTokenExpired by deleting the stored token and retrying the request.

This fixes errors such as the following:

Error: Error fetching changes in zone <CKRecordZoneID: 0x151f6c9c0; ownerName=__defaultOwner__, zoneName=CarsZone>: client knowledge differs from server knowledge

Use UserDefaults.removeObject(forKey:) instead of NSObject.setNilValueForKey() because the latter raises an NSInvalidArgumentException.

[caiyue1993]

Thanks @IngmarStein. It looks good!

Before merging this PR, could you please give us a routine to reproduce this issue? That will help us to check it and help the others to understand this issue. :)

[IngmarStein]

Thinking about it, it may be because I started two SyncEngine objects and they both write to the same prefs key (https://github.com/IngmarStein/Kraftstoff/blob/master/Classes/AppDelegate.swift#L84). #8 will fix this, but I think the error should be handled anyways since there may be other circumstances leading to it.

[caiyue1993]

I've checked it and you are right. We should use removeObject(forKey:) to remove value instead of setNilValueForKey().

But one more question, should we set databaseChangeToken = nil as well when changeTokenExpired error occurs?

[IngmarStein]

It sure looks like, although this can be optimized by passing the information to retryOperationIfPossible if the operation concerns the database or a zone to only reset the respective token, not both.

[caiyue1993]

Agree with you. But I haven't found a appropriate way to determine which operation is running into this error. I don't want to import a global variable to fix this issue. Setting both token to nil practically works.
(For the successors, this page could help you to know more about changeTokenExpired error)