nginx处理http请求流程
http://blog.itpub.net/15480802/viewspace-1393025/监听套接字ngx_listenting_t->fd由获取accept_mutex的worker进程加入epoll监控，其handler为ngx_event_accept；注：每个fd赋予一个ngx_connection_t，且c->read->handler = ngx_event_acc
