Fix issues with prefetch ring buffer resize

[knizhnik]

Problem

See https://neondb.slack.com/archives/C04DGM6SMTM/p1732110190129479

We observe the following error in the logs

[XX000] ERROR: [NEON_SMGR] [shard 3] Incorrect prefetch read: status=1 response=0x7fafef335138 my=128 receive=128

most likely caused by changing neon.readahead_buffer_size

Summary of changes

1. Copy shard state
2. Do not use prefetch_set_unused in readahead_buffer_resize
3. Change prefetch buffer overflow criteria

[github-actions[bot]]

5535 tests run: 5317 passed, 1 failed, 217 skipped (full report)

---

Failures on Postgres 14

• test_pull_timeline[True]: release-arm64

  # Run all failed tests locally:
  scripts/pytest -vv -n $(nproc) -k "test_pull_timeline[release-pg14-True]"

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
085f46280e5b5a6eb087f3baa9af38a80c2cde18 at 2024-11-22T15:45:08.778Z :recycle:}