Add support for kafka journalization of the ProvenanceStorageInterface

the new ProvenanceStorageJournal is a proxy ProvenanceStorageInterface
that will push added objects in a swh-journal (typ. a kafka).

Journal messages are simple dicts with 2 keys: id (the sharding key) and
value (a serialiazable version of the argument of the xxx_add() method).

Use the 'kafka' pytest marker for all kafka-related tests (especially
used for tox, see tox.ini).