message: Update for compatibility with realm/realm-object-store#560
  * Update for compatibility with realm/realm-object-store#560.
  realm/realm-object-store#560 removes the `SyncConfig` default
  constructor since it would otherwise allow an invalid `SyncConfig`
  object to be constructed.
diff: @@ -42,11 +42,9 @@ AdminRealmListener::AdminRealmListener(std::string local_root, std::string serve
  m_config.cache = false;
  m_config.path = util::File::resolve("realms.realm", local_root);
  m_config.schema_mode = SchemaMode::ReadOnlyAlternative;
  - m_config.sync_config = std::make_shared<SyncConfig>();
  - m_config.sync_config->stop_policy = SyncSessionStopPolicy::AfterChangesUploaded;
  - m_config.sync_config->user = user;
  - m_config.sync_config->reference_realm_url = server_base_url + "/__admin";
  - m_config.sync_config->bind_session_handler = std::move(bind_callback);
  + m_config.sync_config = std::make_shared<SyncConfig>(user, server_base_url + "/__admin",
  + SyncSessionStopPolicy::AfterChangesUploaded,
  + std::move(bind_callback));
  }
  void AdminRealmListener::start(std::function<void(std::vector<std::string>)> callback)
  