Improvements and tweaks to latest note handling (resolves #744)

What does this PR do?

This is a "second pass" PR to refine and improve the implementation of latest_note table the following feed.

• Update latest_note in the background, like other logic in CreateNoteService / DeleteNoteService.
• Handle note updates, since it's possible to convert a note to/from a quote.
• Factor out shared logic

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