[#25397] docdb: Added rocksdb_iterator_sequential_disk_reads_factor flag

Summary:
It turned out that for some cases (for example block-based sampling for ANALYZE) we can get better performance if we don't reset readahead state even when scan is not strictly sequential but skipping some amount of data.

Added rocksdb_iterator_sequential_disk_reads_factor flag which allows to relax requirements for treating disk read as sequential for readahead purposes. Set to 1 by default (old behaviour is equivalent of 0).
Jira: DB-14628

Test Plan: ybd --cxx-test readahead_test -n 50 -- -p 1 for release/debug/asan/tsan builds

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D40831