Article create and update slow on web update 1

As of this commit did a bit further investigation with a better tooling and more understanding, notably now we run:

OURBIGBOOK_LOG_DB=1 num run dev-pg

Heroku is definitely slower than local, at around 1 t o2 s on the bit first ten pages:

ourbigbook --web --web-force-render --web-max-renders 10

but local was also rather slow when we have about the same number of articles for the user.

After some improved benchmarking setup, there seem to be two separate causes:

preventing: options.db_provider.fetch_header_tree_ids( on web. It is not necessary as we render the ToC dynamically.
This matters the most for toplevel articles with many descendants.
the other problem we haven't solved yet: the nested index update querries are slow. We don't know how to solve that easily.
Those querries simply update a huge number of rows.
Maybe we could have a fallback mechanism to build that index on the background, and use the tree index temporarily?
Hard call.

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