iwrr: a wrr with always O(1) time and O(n) memory

docs/modules/ngx_http_upstream_iwrr_module.md

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+
+## Name
+
+ngx_http_upstream_iwrr_module.
+
+
+## Introduction
+
+The `IWRR` module is an efficient load balancing algorithm with `O(1)` time complexity, but `IWRR` is no need to incremental initialization.
+
+Compared with Nginx's official `SWRR` algorithm and `VNSWRR`, `IWRR` abandons smoothness on the premise of ensuring the correctness of the weighted load balancing algorithm, ensuring that no matter how the total weight of the cluster changes, `IWRR` space The complexity is always `O(n)`.
+
+## Example
+
+```
+http {
+
+    upstream backend {
+        iwrr; # enable IWRR load balancing algorithm.
+        127.0.0.1 port=81;
+        127.0.0.1 port=82 weight=2;
+        127.0.0.1 port=83;
+        127.0.0.1 port=84 backup;
+        127.0.0.1 port=85 down;
+    }
+    
+    server {
+        server_name localhost;
+        
+        location / {
+            proxy_pass http://backend;
+        }
+    }
+}
+
+```
+
+## Installation
+
+Build Tengine with this module from source:
+
+```
+
+./configure --add-module=./modules/ngx_http_upstream_iwrr_module/
+make
+make install
+
+```
+
+
+## Directive
+
+iwrr
+=======
+```
+Syntax: iwrr [max_init=number]
+Default: none
+Context: upstream
+```
+
+Enable `iwrr` load balancing algorithm. 

docs/modules/ngx_http_upstream_iwrr_module_cn.md

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+
+## 名称 
+
+ngx_http_upstream_iwrr_module.
+
+
+## 介绍
+
+`IWRR`模块是一个高效的负载均衡算法，与`VNSWRR`相同，它具有`O(1)`的时间复杂度，但是`IWRR`不需要执行渐进式初始化操作。
+
+同Nginx官方的加权轮询负载均衡算法及`VNSWRR`相比，`IWRR`在保证加权负载均衡算法正确性的前提下，牺牲了平滑的特点，保证无论集群总权重如何变化，`IWRR`空间复杂度总是`O(n)`的。
+
+## 配置列子
+
+```
+http {
+
+    upstream backend {
+        iwrr; # enable IWRR load balancing algorithm.
+        127.0.0.1 port=81;
+        127.0.0.1 port=82 weight=2;
+        127.0.0.1 port=83;
+        127.0.0.1 port=84 backup;
+        127.0.0.1 port=85 down;
+    }
+    
+    server {
+        server_name localhost;
+        
+        location / {
+            proxy_pass http://backend;
+        }
+    }
+}
+
+```
+
+## 安装方法
+
+在Tengine中，通过源码安装此模块：
+
+
+```
+
+./configure --add-module=./modules/ngx_http_upstream_iwrr_module
+make
+make install
+
+```
+
+
+## 指令描述
+
+iwrr
+=======
+```
+Syntax: iwrr
+Default: none
+Context: upstream
+```
+
+在upstream里面启用 `iwrr` 加权轮询算法。

modules/ngx_http_upstream_iwrr_module/config

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+ngx_addon_name=ngx_http_upstream_iwrr_module
+HTTP_UPSTREAM_IWRR_SRCS="$ngx_addon_dir/ngx_http_upstream_iwrr_module.c"
+
+if test -n "$ngx_module_link"; then
+    ngx_module_type=HTTP
+    ngx_module_name=$ngx_addon_name
+    ngx_module_deps=
+    ngx_module_srcs="$HTTP_UPSTREAM_IWRR_SRCS"
+
+    . auto/module
+else
+    HTTP_MODULES="$HTTP_MODULES ngx_http_upstream_iwrr_module"
+    NGX_ADDON_SRCS="$NGX_ADDON_SRCS $HTTP_UPSTREAM_IWRR_SRCS"
+fi