feat: Put request rate limit at application server (#3857)

* feat: Put request rate limit at application server

This PR contains implementation for a blanket rate limiter at
application server level. Currently the allowed throughput is set high
only to be changed later as per the actual data gathered.

* Simplify implementation

1. Remove shutdown handler to purge rate limiter keys
2. Have separate keys for default and custom(route-based) rate limiters
3. Do not kill default rate limiter because it is not needed anymore due
   to (2) above

* Set 60s as default for rate limiting window

* Fix env types

package.json

@@ -161,6 +161,7 @@
     "query-string": "^7.0.1",
     "quoted-printable": "^1.0.1",
     "randomstring": "1.1.5",
+    "rate-limiter-flexible": "^2.3.7",
     "raw-loader": "^0.5.1",
     "react": "^17.0.2",
     "react-avatar-editor": "^11.1.0",