refactor: update CBlockIndex::nChainTx to be uint64_t #24912

There was a comment with the CBlockIndex::nChainTx data member: "Change to 64-bit type before 2024 (assuming worst case of 60 byte transactions)."

As of when this PR was created, there were only around 750,000,000 transactions in the main best chain. That's only in the ballpark of 20% of the range of a 32-bit unsigned int. So, you might think that this is early. But, since I was looking up the references and already doing most of the work, I figured that I'd propose it now. In the least this commit resolves and removes a comment that might otherwise scare more people into doing the same research that I did, thus saving people time.

I went through all the references to this member and made updates to make the data types line up better where there were differences.

I also removed a small amount of dead code from SnapshotMetadata, see (https://github.com/bitcoin/bitcoin/pull/21681#issue-858110318).

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24952 (rpc: Add sqlite format option for dumptxoutset by dunxen)
• #24804 (Sanity assert GetAncestor() != nullptr where appropriate by aureleoules)
• #21603 (log: Mitigate disk filling attacks by rate limiting LogPrintf by dergoegge)

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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🐙 This pull request conflicts with the target branch and needs rebase.

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closing since there appears to be no interest in this set of changes