Basically callbre means that we call replicaset with two additional parameters:
    1) prefer_replica = true. We can use read from replicas because
        transactional consistency is out of Sbroad scope. Basically we
        do not care about data consistency at all.
    2) balance = true. This parameters enables internal vshard load
        balancing between replicaset nodes in round-robin fashion,
        with a preference for replicas if prefer_replica=true is also set