inventory DB:
-------------
 default loc:
    [TARGET:foo]'s "inventory_db" key, or
    "$ENV{HOME}/.brackup-target-$name.invdb";

 mapping:

   pchunk->inventory_key   --->  join(" ", $schunk->backup_digest, $schunk->backup_length))
   <dig>;to=<rcpt>
   <dig>;raw


Digest DB:
----------
  default loc:
    [SOURCE:foo]'s 'digestdb_file' key, or
    "$SOURCE_DIR/.brackup-digest.db"

 table:  "digest_cache"

    <cache_key> => "sha1:xxxxxxxxx"

 cache_key ::=
    [<root>]<path>:<ctime>,<mtime>,<size>,<ino>