Release v19.91.0

Added

• The kernel accepts appends with an idempotency key (AppendOnce, AppendManyOnce and their named-tags variants): a retried append with the same key and content is reported as already appended instead of writing the event again, and the same key with different content is refused with a conflict. Clients can detect support through the idempotent-appends kernel capability (part of #4809)