Add /_stats endpoint

Also, remove ingest_stats worker and use the cache instead.

package.json

@@ -16,7 +16,6 @@
     "date-fns": "^1.28.1",
     "express": "^4.15.2",
     "gunzip-maybe": "^1.4.0",
-    "http-client": "^4.3.1",
     "invariant": "^2.2.2",
     "isomorphic-fetch": "^2.2.1",
     "mime": "^1.3.6",
@@ -35,7 +34,8 @@
     "sri-toolbox": "^0.2.0",
     "tar-fs": "^1.15.2",
     "throng": "^4.0.0",
-    "validate-npm-package-name": "^3.0.0"
+    "validate-npm-package-name": "^3.0.0",
+    "warning": "^3.0.0"
   },
   "devDependencies": {
     "autoprefixer": "6.7.2",