change(scan): Store one transaction ID per database row, to make queries easier

[teor2345]

Motivation

We want technical users to be able to use ldb to query the scanner database. This means we need one transaction ID per row.

Close #8050.

PR Author Checklist

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• Will the PR name make sense to users?
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Complex Code or Requirements

This changes the format from:
ScannerKey | Height -> Vec<transaction::Hash> to:
ScannerKey | Height | TransactionIndex -> Option<transaction::Hash>.

None can be used to represent birthday heights and scanner progress in #8022.

This format isn't ideal, because handing None is harder to code and review. We can change it to use a custom enum later, or track keys, birthdays, and progress in a separate column family. But that's for after the MVP.

Solution

• Change the scanner database format (see above)
• Change the serialization formats
• Update the major version to 1
• Update format docs

Testing

I updated the existing tests where needed, and they pass.

Review

This is blocking finishing off the scanner, so it seems like a high priority for review.

Reviewer Checklist

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• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
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Follow Up Work

• feat(scanner): Start scanning where it left off last time #8022
• Document how to run and query the results of the blockchain scanning MVP #8046
• re-do test(scan): Scan blocks containing ZECpages transactions with the ZECpages shielded key #7813