A Migration object is passed to you when you migrate your database from one version
to another. It contains the properties for the Realm before and after the migration.
After the migration is complete, newRealm will become the authoritative version of
The signature of a callback that will be executed while the Realm is opened asynchronously with Realm.open.
This is the registered onProgressCallback when calling Realm.open that receives progress notifications while the download is in progress.