fix rejudge, add cache

package.json

@@ -14,7 +14,7 @@
         "koa-body": "^4.1.1",
         "koa-morgan": "^1.0.1",
         "koa-router": "^7.4.0",
-        "koa-send": "^5.0.0",
+        "koa-static-cache": "^5.1.2",
         "lodash": "^4.17.15",
         "markdown-it": "^10.0.0",
         "markdown-it-katex": "^2.0.3",