添加事件到epoll

当客户端刚刚连接上时候，则关注读事件：

c->read->handler = ngx_read_handler;
if(ngx_handle_read_event(c->read, NGX_CLEAR_EVENT)== NGX_ERROR) {
       return NGX_ERROR;
}  

给客户端发送数据未发送完，则关注写事件：

c->write->handler = ngx_write_handler;
if(ngx_handle_write_event(c->write,0) == NGX_ERROR) {
       return NGX_ERROR;
}