page_service: unit-test batching logic

[problame]

          It would be really nice if we could unit test this batching logic (doesn't have to happen in this PR - gone over it enough times to be certain there's nothing obvious). `self` usage isn't too bad here, but we would need to hide `PostgressBackend` behind an interface where we can mock `read_message`. Something like that would be useful in other places too.

Originally posted by @VladLazar in #9792 (comment)

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Consider extracting the merging condition check into a function that can be unit-tested

#9851 (comment)