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Straighten out the fetch plan for feed stories
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See T8575 for a brief mention. Feed/notifications have a tangled mess of a data fetch plan right now:

  • We load all the stories.
  • We load all the primary objects + transactions.
    • I believe ObjectQuery sometimes misses transactions (incorrectly discards them in a way that seems "at random", likely causing T8525).
    • Loading the transactions reloads many objects:
      • Transactions load their primary object, first.
      • I believe ObjectQuery frequently misses the workspace cache.
      • Transactions then load all their handles.
        • This should be off by default.
        • This should use handle pools.
        • I think this misses the workspace cache, too.
  • Now, we load all the required handles.
    • We already did a lot of this work.
    • This misses all the caches.
    • The earlier handle loading should happen here instead.
    • All of this should use handle pools.

In some cases, this ends up looking like: "Load transaction > load object, return; load handle > load object, return; load object; return; Load handle > load object, return". And I caught 6 loads on one profile instead of "only" 5, so that's not even the entire story.

Some of this is relatively easy to disentangle, some of it is a bit messier. Particularly, making transaction query default to not loading handles may be a bit of a mess.

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This seems to be stable now, I don't immediately see any more ridiculously dumb slow stuff in the /feed/ profile. Remarkup handle fetches are still a bit loopy but that's a topic for another task.