Refactor the generate_storage_from_git dataset creation tool
use the newly introduced swh.journal StreamJournalWriter to generate the msgpack-file with the content of the storage serialized, instead of reimplementing a similar logic by hand.
Doing so, the msgpack file format is a little bit different, so current datasets need to be regenerated.
From the 'consumer' side (the fill_storage fixture), simply use swh.storage's process_replay_objects (once again, instead of reinventing the wheel).
This is needed because we will need to handle much more model object types for testing the origin-revision layer.
Migrated from D5891 (view on Phabricator)