Support load-balancing handler

[xfeep]

A load-balancing handler can implement a customized load-balancing algorithm. e.g.

upstream backend {

    load_balancer_type java;
    load_balancer_name example.MyIpHashLoadBalanceHandler;
    load_balancer_property total 2;

    server backend1.example.com;
    server backend2.example.com;
    server backend3.example.com down;
    server backend4.example.com;
}