[sonic-swss-common] Update submodule

Includes sonic-swss-common commits:
```
71dc350 2021-01-07 | Lower the log level for outdated key for SubscriberStateTable notification (sonic-net#441) [Qi Luo]
7e40582 2021-01-08 | Add boost dependencies (sonic-net#442) [Ze Gan]
30a8ddf 2021-01-05 | Change DBConnector::hgetall return type from map to unordered_map (sonic-net#440) [Qi Luo]
021108d 2021-01-02 | MCLAG Enhancements per HLD sonic-net/SONiC#596 (sonic-net#405) [Praveen-Brcm]
54996fc 2021-01-02 | Implement ConfigDBConnector and ConfigDBPipeConnector in C++ (sonic-net#437) [Qi Luo]
8286525 2020-12-27 | Simply refactor DBConnector hgetall() [Qi Luo]
6d1d33b 2020-12-27 | Fix RedisTransactioner: handle empty deque [Qi Luo]
624e0b8 2020-12-26 | Move complex class constructor as explicit, and fix several mistaken copy constructor usage [Qi Luo]
3b983f9 2020-12-30 | [ci]: add timeout to 180 minutes for arm build (sonic-net#439) [lguohan]
f2e4210 2020-12-29 | Add utility for string and redis (sonic-net#434) [Ze Gan]
7a885fd 2020-12-29 | [build]: add build check for arm64 and armhf (sonic-net#436) [lguohan]
47bccc4 2020-12-24 | Add missed vector header to rediscommand.h (sonic-net#435) [Ze Gan]
```

Author: qiluo-msft