db: warn about legacy chainstate table format #35467

Problem: Chainstates created by pre-29 nodes can contain thousands of files from the old 2 MiB LevelDB table target. Now that the mmap limit dropped back to 1000 and seek compaction was disabled, continuing IBD from such a chainstate can leave many reads on the non-mmap path until the database is compacted. A current 32 MiB-layout chainstate had 6 matching small files, while pre-29 chainstates can have more than 6000, so a coarse threshold gives users a warning without flagging normal current layouts.

Fix: Warn at startup when the chainstate database has more than 100 small files and point users to -forcecompactdb.

Fixes #35457

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35467.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34937 (Fix startup failure with RLIM_INFINITY fd limits by Sjors)
• #34132 (coins: drop error catcher, centralize fatal read handling by l0rinc)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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