data must not be into the buffers table

If data is in the buffers table then the data is not
deduplicated... So we store the checksums that compose
a version of a file and we store separatly the checksum
and th corresponding data.
Storing data's buffer and its size
Storing the size of the data into the data table ?

Retrieving all file informations for the structures
in order to be able to store them into the database.
New option in the configuration file

db-cache-name should be a filename of
a database where to store data and
meta data of files as described in
README.md