[sonic-swss-common] Update submodule (#6686)

Includes below commits
```
9e91e0d 2021-02-04 | Fix: RedisClient.set() should return bool (#454) [Qi Luo]
244a0bf 2021-02-04 | [swig] Add missing functions for ConfigDBConnector (#453) [Qi Luo]
e659698 2021-01-22 | [dbconnector] Make sure json is declared as non null object (#452) [Kamil Cudnik]
```