Implement Reset-Metadata Helper

[jleandroperez]

Whenever there's a PSC Model Migration error, the database will not be accessible, and Simperium will be initialized with a blank Core Data storage.

This, as a result, will leave Simperium in an inconsistent state: the internal metadata structures will not get nuked, and thus, after authentication, it's possible that the library won't download the database again.

Implement a helper method, to be called during an eventual PSC migration issue, that would help ensure metadata consistency.