cleanup(db): Give forward and reverse database iterators explicit names

[teor2345]

Motivation

It was really tricky to use our database API in #8050, so I wanted to make iterator directions clearer.

This is different from the Rust Iterator trait, but that's probably ok as long as it's more explicit.

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Specifications

RocksDB has forward and reverse iterators, we use our own generic wrappers for them.

Solution

• Rename iterators to forward, and remove extra boolean arguments
• Ignore a clippy lint

Testing

This is covered by existing tests.

Review

This is a low priority cleanup, but it might be useful for some of the scanner work.

Please double-check that:

• false is forward
• true is reverse

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