refactor: unit/auth to gocase

[tisonkun]

This closes #829.

This refers to #753. It contains changes for src/redis_connection.cc.

go-redis writes:

func (c *baseClient) initConn(ctx context.Context, cn *pool.Conn) error {
	// ...
	if err := conn.Hello(ctx, 3, username, password, "").Err(); err == nil {
		auth = true
	} else if !strings.HasPrefix(err.Error(), "ERR unknown command") {
		return err
	}
	// ...
}

But in src/redis_connection.cc we firstly check AUTH and later check unknown command so the else if branch hitted and return wrong err. I try to resolve it in Kvrocks scope but unsure whether it's the Redis manner.

cc @git-hulk @vmihailenco

[git-hulk]

LGTM