基于libuv的TCP echo-server

学习libuv库，写TCP echo-server

大部分代码都来自libuv源码test目录下echo-server.c文件。

我自己添加了一点代码，通过uv_read_start函数更改read_cb函数

使得第奇数次客户端输入一行文字后，服务端原样返回，

偶数次输入一行文字，服务端将输入的一行文字逆序后返回

结果如下：

cheng@debian:~$ telnet 127.0.0.1 7890
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
helo
helo
helo
oleh
00000111111
00000111111
00000111111
11111100000

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通过echo-server这个程序，可以学习怎么利用libuv，像完成原始socket网络编程那样，

创建socket, bind, listen, accept过程。

只是在异步，非阻塞的方式编程时，需要通过事先为某个事件发生时应该调用的回调

在echo-server程序中，通过uv_listen函数注册连接回调函数

uv_read_start注册读函数

uv_write注册写函数

在头文件uv.h看这三个函数的函数原型，可以很清楚的知道每个参数的意义

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修改过的echo-server.c代码：

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