ETag —— http缓存和静态优化

Cache-Control和ETag ：https://www.jianshu.com/p/3e2afe089e11

ETag详解： https://blog.csdn.net/xmloveth/article/details/52291760
HTTP协议缓存策略深入详解之etag妙用： https://blog.csdn.net/wanywn060/article/details/83607818