Function wait_for_sync_data 

async fn wait_for_sync_data<'c>(
    sync_update_subscriber: Subscriber,
    transaction: &mut TransactedDb<'c>,
    session: &SessionInfo,
    initial: bool,
    sync_token: &mut SyncToken,
    filter_definition: Option<&FilterDefinition>,
    timeout: Duration,
) -> Result<Option<SyncData>, InternalServerError>

Given a [arminius_lib_nats::Subscriber] instanced from sync_data_subscriber, wait until it receives a NATS message about a new sync response.

Then, it checks in the database if there are any new data to synchronize whenever a new message is received by that subscriber, using a call to fetch_sync_data.