[MultiDB]:sonic-swss replace old API with new APIs

[Pan-XT]

What I did
Replace DBConnector old APIs with new APIs
Why I did it
Enable Multi DB

I implemented similar changes in #1138

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Pan-XT / name: PanXuntao (d1d8ab0, 0b39af7)

[eddieruan-alibaba]

Cherry picked to Phoenix wing

eddieruan-alibaba@c065418

[eddieruan-alibaba]

Verified in Phoenix wing's daily sanity.

http://phoenixwing.com.cn/vsonic

[eddieruan-alibaba]

Validated with phoenix wing

admin@PE3:$ ls -l /etc/sonic/enable_multidb
-rw-r--r-- 1 root root 0 Oct 11 01:46 /etc/sonic/enable_multidb
admin@PE3:$ ps aux | grep redis
message+ 990 0.8 0.5 86764 39228 pts/0 Sl 06:28 7:06 /usr/bin/redis-server 127.0.0.1:6379
message+ 991 0.8 0.2 71404 21248 pts/0 Rl 06:28 6:50 /usr/bin/redis-server 127.0.0.1:6380
message+ 992 0.3 0.2 64748 17384 pts/0 Sl 06:28 2:28 /usr/bin/redis-server 127.0.0.1:6381
message+ 994 2.9 0.3 71404 24000 pts/0 Sl 06:28 23:44 /usr/bin/redis-server 127.0.0.1:6382
message+ 996 0.1 0.2 62188 16904 pts/0 Sl 06:28 1:22 /usr/bin/redis-server 127.0.0.1:6383

This PR is a minor fix to update some old api to use new api.