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Before this PR, getpage requests wouldn't hold the `applied_gc_cutoff_lsn` guard until they were done. Theoretical impact: if we’re not holding the `RcuReadGuard`, gc can theoretically concurrently delete reconstruct data that we need to reconstruct the page. I don't think this practically occurs in production because the odds of it happening are quite low, especially for primary read_write computes. But RO replicas / standby_horizon relies on correct `applied_gc_cutofff_lsn`, so, I'm fixing this as part of the work ok replacing standby_horizon propagation mechanism with leases (LKB-88). The change is feature-gated with a feature flag, and evaluated once when entering `handle_pagestream` to avoid performance impact. For observability, we add a field to the `handle_pagestream` span, and a slow-log to the place in `gc_loop` where it waits for the in-flight RcuReadGuard's to drain. refs - fixes https://databricks.atlassian.net/browse/LKB-2572 - standby_horizon leases epic: https://databricks.atlassian.net/browse/LKB-2572 --------- Co-authored-by: Christian Schwarz <Christian Schwarz>