(no) Pre batch processing task state (Option 3)

[dwsutherland]

closes #6567, closes #6860, closes #6859

Tasks batched (for DB read) during a graph walk, and processed at the end of that walk.

Window resizes typically include many walks (for each active task), so the DB will have that many calls.
However, no state-less tasks will exist.

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• I have read CONTRIBUTING.md and added my name as a Code Contributor.
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• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
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• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

This seems like the right sort of fix (i.e, get the task status right first time).

If I'm reading this correctly:

• Before we were performing all the DB lookups at once.
• This change splits it up into smaller batches.

If I'm understanding correctly, the root cause is that the data store is trying to push out incomplete deltas before it has actually gathered all of the required information. This (only?) happens on reload.

If so, could we not just delay sending the deltas (after reload) until they are complete (i.e, after we have finished the first walk)?