Refactor iter_store_keys

[MitchTurner]

Description

Was looking into:

                // We have to clone the prefix and start, because we need to pass them to the iterator
                // if someone finds a solution without making it a vec, feel free to contribute :)
                let column = column.id();
                let prefix = prefix.map(|prefix| prefix.to_vec());
                let start = start.map(|start| start.to_vec());

and did some refactoring to get to the bottom of the problem.

I don't know if there is a solution unless we decide to change the signature of iter_store_keys to something like:

    fn iter_store_keys<'a>(
        &'a self,
        column: Self::Column,
        prefix: Option<&'a [u8]>,
        start: Option<&'a [u8]>,
        direction: IterDirection,
    ) -> BoxedIter<KeyItem>;

But that's not realistic in this PR.

Might as well keep the refactoring because I feel like it's cleaner.

[AurelienFT]

Thank you for that, yeah I'm sad we can't do better without breaking the trait but it's logic :/