Добавить возможность проактивных проверок Upstreams

[chudinanton]

Просьба добавить возможность проактивных проверок Upstreams, как это реализовано в Nginx Plus и Tengine от Alibaba "Proactive health checks of upstream servers can be performed." https://github.com/alibaba/tengine

	upstream cluster1 {
		# simple round-robin
		server 192.168.0.1:80;
		server 192.168.0.2:80;

		check interval=3000 rise=2 fall=5 timeout=1000 type=http;
		check_http_send "HEAD / HTTP/1.0\r\n\r\n";
		check_http_expect_alive http_2xx http_3xx;
	}

	upstream cluster2 {
		# simple round-robin
		server 192.168.0.3:80;
		server 192.168.0.4:80;

		check interval=3000 rise=2 fall=5 timeout=1000 type=http;
		check_keepalive_requests 100;
		check_http_send "HEAD / HTTP/1.1\r\nConnection: keep-alive\r\nHost: foo.bar.com\r\n\r\n";
		check_http_expect_alive http_2xx http_3xx;
	}

Документация к модулю: https://github.com/alibaba/tengine/blob/master/docs/modules/ngx_http_upstream_check_module.md