[MM-16562] Migrate User.GetUnreadCountForChannel to Sync by default (#11604)

* Migrate User.GetUnreadCountForChannel to Sync by default

* Remove else block from GetUnreadCountChannel in user_store.go

* Return nil value for count when an error occurs in GetUnreadCountForChannel

* Fix linting issue in user_store.go

store/sqlstore/user_store.go

@@ -1143,14 +1143,12 @@ func (us SqlUserStore) GetUnreadCount(userId string) (int64, error) {
 	return count, nil
 }
 
-func (us SqlUserStore) GetUnreadCountForChannel(userId string, channelId string) store.StoreChannel {
-	return store.Do(func(result *store.StoreResult) {
-		if count, err := us.GetReplica().SelectInt("SELECT SUM(CASE WHEN c.Type = 'D' THEN (c.TotalMsgCount - cm.MsgCount) ELSE cm.MentionCount END) FROM Channels c INNER JOIN ChannelMembers cm ON c.Id = cm.ChannelId AND cm.ChannelId = :ChannelId AND cm.UserId = :UserId", map[string]interface{}{"ChannelId": channelId, "UserId": userId}); err != nil {
-			result.Err = model.NewAppError("SqlUserStore.GetMentionCountForChannel", "store.sql_user.get_unread_count_for_channel.app_error", nil, err.Error(), http.StatusInternalServerError)
-		} else {
-			result.Data = count
-		}
-	})
+func (us SqlUserStore) GetUnreadCountForChannel(userId string, channelId string) (int64, *model.AppError) {
+	count, err := us.GetReplica().SelectInt("SELECT SUM(CASE WHEN c.Type = 'D' THEN (c.TotalMsgCount - cm.MsgCount) ELSE cm.MentionCount END) FROM Channels c INNER JOIN ChannelMembers cm ON c.Id = cm.ChannelId AND cm.ChannelId = :ChannelId AND cm.UserId = :UserId", map[string]interface{}{"ChannelId": channelId, "UserId": userId})
+	if err != nil {
+		return 0, model.NewAppError("SqlUserStore.GetMentionCountForChannel", "store.sql_user.get_unread_count_for_channel.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+	return count, nil
 }
 
 func (us SqlUserStore) GetAnyUnreadPostCountForChannel(userId string, channelId string) (int64, *model.AppError) {