[mdc][leo_storage][docs] deleted objects can be left in the destination cluster

[mocchira]

This can happen when a MDC replication succeeded but its corresponding local replication in the destination cluster failed and queue files got lost for some reason. http://leo-project.net/leofs/docs/admin_guide/admin_guide_4.html#recover-cluster-command help recover objects to be created|updated on the destination side but may not work to be deleted in case the objects deleted on the source side already got compacted (tombstones gone).

The workaround is

• reset(delete all objects or set up the cluster from scratch) the destination cluster
• recover-cluster

The above workaround sysadmin have to take can be improved by checking the objects in the destination side that don't be touched during the recover process and regard those as deleted. Since there are much other factors we have to take care for making it works properly, The design phase needed.