Major Description
By fixing HBASE-10329, we know it's possible an asyncer's writes could have been synced by other asyncer before it starts to do the sync. We distinguish this case and handle accordingly when writer is null, but only for when writer is null.
When writer is not null(the much more typical case), we also can check and ignore the sync if asyncer's writes have been synced by other asyncer before doing sync. Since sync is a quite heavy operation(together with the following notifying notifier thread and log roll check) we can have some performance gain by ignoring it for such case.