lib: fix ExpiringMap internal storage growing

Internal storage of ExpiringMap used to store key deletion times was not
cleared after deleting corresponding keys, leading to its indefinite
growth.

Fixes: #413
PR-URL: #414
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Dmytro Nechai <nechaido@gmail.com>

lib/common.js

@@ -128,6 +128,7 @@ class ExpiringMap extends Map {
     this._deletionTimes.forEach((delTime, key) => {
       if (purgeTime - delTime > this._interval) {
         super.delete(key);
+        this._deletionTimes.delete(key);
       }
     });
   }