backup-restore : incremental_backup

[slivne]

Check that incremetal backup works as expected
1. Use a single node
2. Enable incremental_backup
3. Create a keyspace + table
4. Insert data
5. Check that while sstables are flushed - incremental backups are created
6. Run compact -forcing all ssstables to be merged
7. Check that backups holds all the old files and the new compacted file