Cherry-picking the following patches for fixing #93 
```
0a3f87a2 2019-10-15 yiwu@pingcap.com     Temp fix for data loss caused by concurrent flush (#96)
8ac5003c 2019-10-16 zbk602423539@gmail.. merge BackgroundGC with TEST_StartGC (#94)
b9915d9b 2019-10-07 zbk602423539@gmail.. check nullptr (#91)
```