* @brief A helper function used during post-merge insert * * When the inserting leads to overflow, it will split the node to 1 parent * and 2 children. The parent will be merged to its origin parent after * that. This is the function to complete this task. So the param node is * always a 2-node. * * @param dst_node the target node we will merge node to, can be type of * 2-node, 3-node or 4