Fix InMemoryJournalWriter: do not anonymize objects by default
908f0154 introduced and unexpected change of behavior of the InMemoryJournalWriter, making it always anonymize objects. This broke tests for some dependencies (swh-storage).
This fix adds an 'anonymize' (bool) argument to the class constructor (similar to the KafkaJournalWriter).
Migrated from D7970 (view on Phabricator)