* rename variables in testPostStoreGetPostsWithDetails
This helps to clarify the structure of the posts under test.
* clarify and expand existing testPostStoreGetPostsWithDetails assertions
* expand testPostStoreGetPostsWithDetails assertions
This verifies that replies to posts in the window, themselves not in the
window (because of a non-zero offset) are still fetched. They were
previously missing.
* MM-9770: rewrite getParentsPosts to improve performance
See discussion on ~developers-performance, but the basic idea here is to
force the database to use the `PRIMARY` index when fetching posts
instead of trying to filter down by channel and doing a scan.
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