This work was supported by the National Natural Science Foundation of China (Nos. 11861037, 71471020, 51839002), the Hunan Provincial Natural Science Foundation of China (No. 2016JJ1001), and the Scientific Research Fund of Hunan Provincial Education Department (No. 15A003). The author would like to express the sincere appreciation to the editors and anonymous reviewers for their constructive comments and suggestions which helped to improve the present paper.