Issue 6559 - Use Map object for known subscriptions

lib/synchronizer.js

Index: lib/synchronizer.js
===================================================================
--- a/lib/synchronizer.js
+++ b/lib/synchronizer.js
@@ -195,21 +195,21 @@
     if (downloadable.redirectURL &&
         downloadable.redirectURL != downloadable.url)
     {
       let oldSubscription = Subscription.fromURL(downloadable.url);
       subscription.title = oldSubscription.title;
       subscription.disabled = oldSubscription.disabled;
       subscription.lastCheck = oldSubscription.lastCheck;
 
-      let listed = (oldSubscription.url in FilterStorage.knownSubscriptions);
+      let listed = FilterStorage.knownSubscriptions.has(oldSubscription.url);
       if (listed)
         FilterStorage.removeSubscription(oldSubscription);
 
-      delete Subscription.knownSubscriptions[oldSubscription.url];
+      Subscription.knownSubscriptions.delete(oldSubscription.url);
 
       if (listed)
         FilterStorage.addSubscription(subscription);
     }
 
     // The download actually succeeded
     subscription.lastSuccess = subscription.lastDownload = Math.round(
       Date.now() / MILLIS_IN_SECOND
@@ -330,17 +330,17 @@
         request.channel.loadFlags = request.channel.loadFlags |
                                     request.channel.INHIBIT_CACHING |
                                     request.channel.VALIDATE_ALWAYS;
         request.addEventListener("load", ev =>
         {
           if (onShutdown.done)
             return;
 
-          if (!(subscription.url in FilterStorage.knownSubscriptions))
+          if (!FilterStorage.knownSubscriptions.has(subscription.url))
             return;
 
           let match = /^(\d+)(?:\s+(\S+))?$/.exec(request.responseText);
           if (match && match[1] == "301" &&    // Moved permanently
               match[2] && /^https?:\/\//i.test(match[2]))
           {
             redirectCallback(match[2]);
           }