Fix the inmemory journal writer
InMemoryJournalWriter.write_addition() is expected to receive a (frozen) model entity (not a dict) so it makes no sense any more to deepcopy it.
It's not a required fix for the journal by itself, but is necessary for the storage (in which there is the non-copiable LazyContent model object).
Migrated from D2968 (view on Phabricator)