Optimize live/backup replication after failback

Currently if there is master/slave pair configured using replicated journal then after each failback when master is starting, it copies the whole journal directory from slave.
This seems to ineffective as master might have significant part of backups journal before it failed.

If only differences between journals would be transfered then it would be effective especially in case when failback is completed and master starts to synchronize the whole journal back to slave. It copies the whole journal to backup again which is almost the same. This would greatly speed up synchronization of slave and safe network bandwidth.