Add copy-data stage

[karol-kokoszka]

After validation, Scylla Manager must copy SSTables of the source node to the corresponding /keyspace/column_family/upload data directory on the destination node.

If the corresponding /keyspace/column_family/upload data directory doesn't exist, it means that schema is not restored.
Service must stop the restore with the meaningful error message.

To download files from the backup location, SM is using RClone.
It's OK to copy all the data from the source_node node/keyspace/column_family location with one call and then monitor the rclone job progress.