Introduce ownership semantics for our handles

[nirinchev]

The RealmHandle should have the ability to own other handles and close them if necessary. Examples:

1. Session handle - otherwise the session may outlive the Realm preventing deletion.
2. SubscriptionSetHandle - same as 1.
3. Object/List/Results handles when the Realm is unowned, e.g. migrations, initial data.

We can look at the .NET handle logic for inspiration.