Fix sync resource hog

Hypothesis:
On receiving onChanged event of browser.storage.sync, sync will try to
_merge_ the container data and backup the merged data. One of the merging
process is called reconcileSiteAssignments in which sync will remove
assigned site form the delete site list. On each removal sync will tell
assignManager, the one who hold container data locally, to remove it as
well. In turn the assignManager will remove *and then* tell sync to
backup the data. This causes the backup process in sync to be invoked
multiple times, one from the sync process and one for each of removed
assigned site list. To make matters worse, the backup from removal of
assigned site list will run in parallel from each others.

This might fixes #1691.

src/js/background/assignManager.js

@@ -84,13 +84,13 @@ window.assignManager = {
       return;
     },
 
-    async remove(pageUrlorUrlKey) {
+    async remove(pageUrlorUrlKey, shouldSync = true) {
       const siteStoreKey = this.getSiteStoreKey(pageUrlorUrlKey);
       // When we remove an assignment we should clear all the exemptions
       this.removeExempted(pageUrlorUrlKey);
       await this.area.remove([siteStoreKey]);
       const syncEnabled = await this.getSyncEnabled();
-      if (syncEnabled) await sync.storageArea.backup({siteStoreKey});
+      if (shouldSync && syncEnabled) await sync.storageArea.backup({siteStoreKey});
       return;
     },
 

src/js/background/sync.js

@@ -493,9 +493,9 @@ async function reconcileSiteAssignments() {
     await sync.storageArea.getDeletedSiteList();
   for(const siteStoreKey of deletedSiteList) {
     if (Object.prototype.hasOwnProperty.call(assignedSitesLocal,siteStoreKey)) {
-      assignManager
+      await assignManager
         .storageArea
-        .remove(siteStoreKey);
+        .remove(siteStoreKey, false);
     }
   }