Fix update timestamp work item firing too frequently

A reconfiguration would schedule a new work item without cancelling the
old one, resulting in the timestamp updating multiple times in quick
succession (noticeable for statuses <60s old).

Tusker/Views/Status/TimelineStatusCollectionViewCell.swift

@@ -410,6 +410,9 @@ class TimelineStatusCollectionViewCell: UICollectionViewListCell, StatusCollecti
     }
     
     private func doUpdateTimestamp(status: StatusMO) {
+        // if there's a pending update timestamp work item, cancel it
+        updateTimestampWorkItem?.cancel()
+        
         timestampLabel.text = status.createdAt.timeAgoString()
         
         let delay: DispatchTimeInterval?