[buffermgrd] Use unix socket for redis connection

[Junchao-Mellanox]

Why I did this?

Most of the DB connection in buffermgrd is using unix socket. There is only one DB connection which is using TCP socket. It brings a few drawback:

1. TCP socket is less efficiency
2. TCP socket may fail if loopback interface is down. It could cause following error:

Apr 29 16:53:25.586513 sonic ERR swss#buffermgrd: :- main: Runtime error: Unable to connect to redis: Cannot assign requested address

How I did this

Change TCP socket to unix socket

How I verify this

Manual test

[Junchao-Mellanox]

ci passed 4146