Fix the order by problem in the sub-query and possible int32 overflow problem

    - Add ArrayType<int64_t> and MutableArrayType<int64_t> type trait
    - Rewrite the core query for training and prediction to avoid using internal order by
    - Change the type of model from INT4 to INT8
Robust Variance for Cox (Huber-White Sandwich)

Pivotal Tracker: 51528937

Other authors:
    Qian, Hai <hqian@gopivotal.com>
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

Changes:
    - Design, and Implementation of Robust Variance for Cox