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.

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Migrated from D7952 (view on Phabricator)