Idea - shared backup storage + background merges

[Slach]

Think
Shared backup storage
/backup_data/{part_checksum}.tar
/backup_parts/backup_name.json - store part_checksum->part_name
/backup/backup_name/metadata.json
/backup/backup_name/metadata/db/table.json

background cleanup

• read /backup_parts/*.json
• list /backup_data/
• compare lists and delete /backup_data/ for checksums which not exists in /backup_parts/*.json

actually we need file checksum instead of part checksum