Start with minimal prefetch distance to minimize prefetch overhead for exact or limited index scans (#10359)

## Problem

See https://neondb.slack.com/archives/C04DGM6SMTM/p1736526089437179

In case of queries index scan with LIMIT clause, multiple backends can
concurrently send larger number of duplicated prefetch requests which
are not stored in LFC and so actually do useless job.

Current implementation of index prefetch starts with maximal prefetch
distance (10 by default now) when there are no key bounds, so in queries
with LIMIT clause like `select * from T order by pk limit 1` compute can
send a lot of useless prefetch requests to page server.

## Summary of changes

Always start with minimal prefetch distance even if there are not key
boundaries.

Related Postgres PRs:
https://github.com/neondatabase/postgres/pull/552
https://github.com/neondatabase/postgres/pull/551
https://github.com/neondatabase/postgres/pull/550
https://github.com/neondatabase/postgres/pull/549

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

vendor/revisions.json

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