Fixes Xcode 6 warning
Nukes NSManagedObjectContext userInfo usage

Due to Xcode 6 CoreData threading violations, each userInfo call should
be wrapped up by a performBlock call.

This should be executed by every single NSManagedObject that gets
faulted / inserted into any context, and has a considerable performance
impact.

Let’s bring back the old school obc_setAssociatedObject, which was the
default mechanism in Simperium Mk 0.5.0