Deleting large amount of objects at once sometimes fail

[jphribovsek]

We have an entity with couple of NSString/NSNumbers attributes, small objects.
Users can have a large number of those objects, in this case 10,000.
There is an option for users to do a "clear all" and remove all objects at once.
The clear all method loops through all objects in core data and performs a deleteObject: for each of them, then a single save when the loop is over.

• the operation takes a long time to sync, delete one at a time on the server
• the operation eventually stops. So even though the device shows 0 items (all removed from core data), a few thousands items remain on the server, and devices get out of sync. The operation never resumes.

[jleandroperez]

This is caused by #228

[jphribovsek]

In addition to that, the main thread is blocked when performing large bulk delete, it's causing major performance issues.

[jleandroperez]

Let's handle the performance issue in #274.

This fix should not happen anymore (Ref. #228 is fixed, thanks JP!). Plus, now the error handling mechanism has been patched not-to-fail when dealing with deletions.