Improve finding of terminus when shifting items [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Improves querying for the min / max object when we need to shift elements to make space.

We already have the min / max position from the previous query so we just need to find the object with that position instead of querying and sorting all objects again and finding the min / max one.

Related to gitlab-com/gl-infra/production#4283 (closed)

Queries

Before: https://explain.depesz.com/s/hkC7

After: https://explain.depesz.com/s/iPRI

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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