perf: Reuse redis connections where possible (#1157)

* reuse redis connections where possible

* redis -> ioredis

package.json

@@ -95,6 +95,7 @@
     "immutable": "^3.8.2",
     "imports-loader": "0.6.5",
     "invariant": "^2.2.2",
+    "ioredis": "^4.14.1",
     "isomorphic-fetch": "2.2.1",
     "js-search": "^1.4.2",
     "json-loader": "0.5.4",
@@ -138,7 +139,6 @@
     "react-portal": "^4.0.0",
     "react-router-dom": "^5.1.2",
     "react-waypoint": "^9.0.2",
-    "redis": "^2.6.2",
     "rich-markdown-editor": "^9.11.1",
     "sequelize": "^5.21.1",
     "sequelize-cli": "^5.5.0",