Take advantage of an existing update on All-Users to prune any
published comments from drafts. NoteDbUpdateManager takes care of
ensuring that this update is applied before its dependent draft
update.

Deleting aggressively in this way, combined with filtering out
duplicate published/draft comments in ChangeNotes#getDraftComments,
makes up for the fact that updates between the change repo and
All-Users are not atomic.

TODO(dborowitz): We might want to distinguish between deleted
drafts that we're fixing up after the fact by putting them in a
separate commit. But note that we don't care much about the commit
graph of the draft ref, particularly because the ref is completely
deleted when all drafts are gone.