[sonic-swss-common] Update submodule (#8272)

Includes below commits
```
bf8c832 2021-07-22 | Fix DBInterface blocking operations (#505) (HEAD -> 202012, origin/202012) [Qi Luo]
0e9385f 2021-04-21 | [swig] Implement SonicV2Connector.hmset() (#480) [Qi Luo]
76be49f 2021-07-25 | Modify the hardcode separator ":" to variable. (#473) [PJHsieh]
142ae3c 2021-06-23 | Fix config prompt question issue (#500) [xumia]
e7bebe1 2021-06-14 | Fix repo reference issue (#487) [xumia]
```