JournalClient: change the prefix init argument dafault value to None
and give this None value the special meaning of 'use the default value',
i.e. 'swh.journal.objects' (specifically, the value stored as
This is required to make it easier to handle cli arguments and config
values read from the config file. Let the None value be the default
(from the cli config/argument handling) as long as possible and decide
only in the JournalClient what "default" really means.