[Feature#3234][cluster]enhanced load balancing (#3235)

* [Future#3234][cluster]enhanced load balancing
weight-based load balancing algorithm
this close # 3234

* remove useless parameter

* code smell

* load balancing according to work group

* add smooth weight round robin

* remove unused constants

* perfect unit test

* code smell

* code smell

* add work weight config

* fix config error

* add weight docs to readme.md