Don't re-sort already fetched post thread items (#6698)

* Don't reorder already seen posts in PostThread

* Add sorting by generation

* Rip out stable order cache

It doesn't make sense because sort() doesn't call the callback for all A/B pairs, and the server returning a different ordering will cause cache misses which means there'll be no stability anyway.

* Make hotness deterministic per fetched at

* Cache random scores while in thread

* Reorder for clarity

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