Add type annotation for JournalWriter related code
- add a JournalWriterInterface in a new interface.py module,
- add a flush() method to this interface,
- remove (now unused) write_update methods,
- move all type-related code in this module,
- fix InMemoryJournalWriter type annotations.
- remove unused 'privileged' argument from journal writer's write_addition(s) for StreamJournalWriter and InMemoryJournalWriter, and make them consistent with KafkaJournalWriter.
Migrated from D7952 (view on Phabricator)