[new feture] support client-user level rate limit

[funnyAnt]

By configuring max_rate_limit in user properties, achieve the effect of flow control 。Detail config as following:

"users": [
        {
            "user_name": "**",
            "password": "***",
            "namespace": "sysbench_4",
            "rw_flag": 2,
            "rw_split": 1,
            "open_rate_limit": true,
            "max_rate_limit": 1000,
            "max_token_wait_time": 10
        }
    ],