Extraction works.
Keep the storage hash in mbdb_record.

Instead of re-generating the storage hash every time we need it, we can just
generate it once at construction time and save it.
Organize files on their per-domain directories.

Files in an MBDB backup are structured with a "domain" structure. Each domain is
some sort of independant hierarchy and names can overlap between domains, so we
need to keep everything separate.
Improve the output of  command.
Set correct permissions on extracted files.