This implements support for "constant-time" pagination.
Obviously it's not constant-time, but it scales in a sane fashion.

This PR is still not about general performance optimizations. It is the last step before those can start though.

• Fixes Dataframe v2: constant-time pagination #7657
• DNM: requires Dataframe v2: support for clear semantics #7652

Test: check the performance while scrolled down at the bottom of a large dataframe.

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