Empty older articles' status before adding new ones

This makes the "Last Refresh" filter to behave more like it did before
pull request ViennaRSS#1770 (cf. commit bb06760)
Significant difference though: "Last Refresh" refers to the last time
a particular feed was refreshed and got new articles, while previously
it referred to the last time "Refresh All Subscriptions" was triggered.

Vienna/Sources/Fetching/OpenReader.m

@@ -639,6 +639,7 @@ -(void)feedRequestDone:(NSMutableURLRequest *)request response:(NSURLResponse *)
 
             // Here's where we add the articles to the database
             if (articleArray.count > 0) {
+                [refreshedFolder resetArticleStatuses];
                 NSArray *guidHistory = [dbManager guidHistoryForFolderId:refreshedFolder.itemId];
 
                 for (Article *article in articleArray) {

Vienna/Sources/Fetching/RefreshManager.m

@@ -854,6 +854,7 @@ -(void)finalizeFolderRefresh:(NSDictionary *)parameters
 
         // Here's where we add the articles to the database
         if (articleArray.count > 0u) {
+            [folder resetArticleStatuses];
             NSArray *guidHistory = [dbManager guidHistoryForFolderId:folderId];
             for (Article * article in articleArray) {
                 if ([folder createArticle:article

Vienna/Sources/Models/Folder.h

@@ -117,6 +117,7 @@ typedef NS_OPTIONS(NSUInteger, VNAFolderFlag) {
 -(BOOL)createArticle:(Article *)article guidHistory:(NSArray *)guidHistory;
 -(void)removeArticleFromCache:(NSString *)guid;
 -(void)markArticlesInCacheRead;
+-(void)resetArticleStatuses;
 -(NSArray<ArticleReference *> *)arrayOfUnreadArticlesRefs;
 -(NSComparisonResult)folderNameCompare:(Folder *)otherObject;
 -(NSComparisonResult)folderIDCompare:(Folder *)otherObject;

Vienna/Sources/Models/Folder.m

@@ -658,6 +658,21 @@ -(void)markArticlesInCacheRead
   } // synchronized
 }
 
+/* resetArticleStatuses
+ * iterate through the cache and empty the articles status
+ */
+-(void)resetArticleStatuses
+{
+    for (NSString * guid in self.cachedGuids) {
+        Article * article = [self.cachedArticles objectForKey:guid];
+        if (article) {
+            [article beginContentAccess];
+            article.status = ArticleStatusEmpty;
+            [article endContentAccess];
+        }
+    }
+}
+
 /* arrayOfUnreadArticlesRefs
  * Return an array of ArticleReference of all unread articles
  */