txindex: allow running in pruned mode #36002

After #35531, each txindex entry now points to the block hash containing the tx instead of the file location of the block. The block hash is then used to find the file position of the block to read the tx from. This allows us to determine the hash of the containing block even if the block is later pruned, and also determine the location of the block on disk if it is later downloaded again via getblockfrompeer.

This PR enables maintaining a txindex while pruned. If a txid lookup tries to read a tx from a pruned block, the hash of the missing block is surfaced to the caller.

If desired, a user may call getblockfrompeer using the returned hash to download the block, then try again successfully. The downloaded block will be added to the front of the block queue, so won't be pruned again for at least 288 blocks. Of course a larger prune setting is desired if lots of historical blocks will be read, and there are privacy issues if the user is looking up their own transactions. Ideally the blocks can be fetched from a trusted node or from random peers with decoy blocks fetched occasionally.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36002.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34729 (Reduce log noise by ajtowns)
• #34132 (coins,dbwrapper: remove error catcher, make point-read failures fatal by l0rinc)
• #31260 (scripted-diff: Type-safe settings retrieval by ryanofsky)
• #24230 (indexes: Stop using node internal types and locking cs_main, improve sync logic by ryanofsky)

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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🚧 At least one of the CI tasks failed. _{Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/32082864144/job/95549090994} _{LLM reason (✨ experimental): CI failed because IWYU reported an include issue (“Failure generated from IWYU”) in src/index/txindex.cpp.}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

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Big concept ACK \:D/