**Acknowledgements**

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51621063, 51321003, 51571157, 51322104 and 51201123) and the 111 Project of China (B06025). GL thanks the support from the National Science Find for Distinguished Yong Scholars. JYZ is grateful for Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2015JM5158) and China Postdoctoral Science Foundation (2016M590940) for part of financial support.
