Move metadata writes to be atomic with datastore writes + improve performance

[mikejohnstn]

Currently there is still metadata stored in NSUserDefaults (e.g. for pending changes). It's possible, if an app crashes, that this could fall out of sync with objects that have been written to the datstore.

Ideally, data writes and associated metadata should be written atomically. This will probably involve dynamically creating an entity in the persistent store for all Simperium metadata instead of using NSUserDefaults or Core Data UserInfo.

[mikejohnstn]

This pull request will get us part of the way there, but may not achieve atomicity just yet: #141

[jleandroperez]

@mikejohnstn atomicity in the Memory Footprint branch is (almost) trivial. But the performance impact was considerable, when dealing with "huge" loads.

[jleandroperez]

Closing in favor of Issue #442