Client reset during async open may lead to a fatal error

[sync-by-unito]

When a realm is opened asynchronously (Realm::get_synchronized_realm()), the new schema is set when download completes (as a result of RealmCoordinator::get_unbound_realm() being called). If the new schema contains extra tables, this can be a problem in case of a client reset.
There are two callbacks (notify_before_client_reset & notify_after_client_reset) in case of a client reset, which if set, are called with an instance of a frozen realm. The frozen realm is opened with the new schema, which fails validation if it not a subset of what actually exists:

'Unsupported schema changes were made by another client or process:

• Class '<table_name>' has been removed.'