max_conns
限制每台server的连接数，用于保护避免过载起限流作用
测试参考配置如下：
# worker进程设置1个，便于测试观察成功的连接数
worker_process 1;
upstream tomcats {
server 192.168.206.129:8080 max_conns=2;
server 192.168.206.130:8080 max_conns=2;
server 192.168.206.131:8080 max_conns=5;
}
slow_start
商业版，需要付费
配置参考如下：
upstream tomcats {
server 192.168.206.129:8080 weight=6 slow_start=60s;
server 192.168.206.130:8080 weight=2;
server 192.168.206.131:8080 weight=2;
}