InferSystemTZ reads system timezone from `TZ`, the path of the soft link of `/etc/localtime`. If both of them are failed, system timezone will be set to `UTC`. It is exported because we need to use it during bootstap stage. And it should be only used at that stage.
LoadLocation loads time.Location by IANA timezone time.
SetSystemTZ sets systemTZ by the value loaded from mysql.tidb.
SystemLocation returns time.SystemLocation's IANA timezone location. It is TiDB's global timezone location.
Zone returns the current timezone name and timezone offset in seconds. In compatible with MySQL, we change `SystemLocation` to `System`.