make all the add_xxx methods take an Iterable as argument. Also extract
the conditional execution of the underlying journal method calls in a
dedicated methof to prevent repeating the `if self.journal` all over the
Ensure all the storage code that uses this JournalWriter class are updated
Also in this diff:
- Improve a bit the exception message of JournalWriter.content_update
- Add missing return annotations on JournalWriter methods
Depends on D3137.