Provides hooks for indicating a migration is running

- Initializes the stack on a background thread first, checking to see if a migration is necessary. If so, the willMigrate hook is called, allowing you to show a "Migration in Progress" type alert.
- The migration is performed implicitely by setting up the store coordinator
- After the migration the didMigrate hook is called.
- Last, everything is torn down and re-created per-usual on the main thread and the initializationComplete hook is called.