This work is supported by the National 973 Program of China (2015CB352202), the National Science Foundation of China (61272047, 91318301, 61321491), the Collaborative Innovation Center of Novel Software Technology and Industrialization, and the program A for Outstanding PhD candidate of Nanjing University. The authors are grateful for the constructive discussions from MathOverflow on the calculations in Section 4. Finally, the insightful comments of the anonymous reviewers greatly improved this paper. Yu Huang is the corresponding author.