Make 'swh db init' fill the dbversion table

for this feature to work properly, the backend datastore package must
provide the 'get_datastore' factory function in its root namespace and
the datastore instance must provide a 'get_current_version()' method.

This also means the canonical cli usage for the `swh db init` is now
using the config file (via `--config-file` or `SWH_CONFIG_FILENAME`)
instead of giving only the db cnx string (`--db-name`), so that the
backend datastore class can be instanciated using the `get_datastore()`
factory function.

However a '--initial-version' cli option has been added to 'swh db init'
to make it easier, especially for unit tests.