Implement method prepare() which would initialize
variables required to store package contents and
some other stuff. prepare() method also load the
last snapshot, get the known versions from that
previous snapshot and then remove those known
versions from the list of all version present in key
tarball in the metadata of the package.

To check for known versions, the loader rely on
a particular field of the metadata. The name of
that field is stored variable compare_field(eg sha).

After removing the already archived versions from
the available method a generator object new_versions
is instantated to process each new version seprately.

Depends on D1694